diff --git a/app/auth/sign-in/page.tsx b/app/auth/sign-in/page.tsx
index b7c0682..d93a6ad 100644
--- a/app/auth/sign-in/page.tsx
+++ b/app/auth/sign-in/page.tsx
@@ -5,28 +5,80 @@ import { authClient } from "@/lib/auth-client";
 import { useRouter } from "next/navigation";
 import Link from "next/link";
 
+const socialProviders = [
+  {
+    id: "google",
+    name: "Google",
+    subtitle: "Platzhalter",
+    icon: "G",
+  },
+  {
+    id: "apple",
+    name: "Apple",
+    subtitle: "Platzhalter",
+    icon: "",
+  },
+];
+
 export default function SignInPage() {
   const router = useRouter();
-  const [email, setEmail] = useState("");
+  const [identifier, setIdentifier] = useState("");
   const [password, setPassword] = useState("");
   const [error, setError] = useState("");
   const [magicLinkMessage, setMagicLinkMessage] = useState("");
+  const [socialMessage, setSocialMessage] = useState("");
   const [loading, setLoading] = useState(false);
   const [magicLinkLoading, setMagicLinkLoading] = useState(false);
 
+  const toGermanAuthError = (message?: string) => {
+    if (!message) {
+      return "Anmeldung fehlgeschlagen. Bitte versuche es erneut.";
+    }
+
+    const normalized = message.toLowerCase();
+
+    if (normalized.includes("invalid") || normalized.includes("credentials")) {
+      return "E-Mail/Username oder Passwort ist nicht korrekt.";
+    }
+
+    if (normalized.includes("verify") || normalized.includes("verification")) {
+      return "Bitte bestätige zuerst deine E-Mail-Adresse.";
+    }
+
+    if (normalized.includes("username")) {
+      return "Username oder Passwort ist nicht korrekt.";
+    }
+
+    return "Anmeldung fehlgeschlagen. Bitte prüfe deine Eingaben.";
+  };
+
   const handleSignIn = async (e: React.FormEvent) => {
     e.preventDefault();
     setError("");
+    setMagicLinkMessage("");
     setLoading(true);
 
     try {
-      const result = await authClient.signIn.email({
-        email,
-        password,
-      });
+      const trimmedIdentifier = identifier.trim();
+      const isEmailInput = trimmedIdentifier.includes("@");
+
+      if (!trimmedIdentifier) {
+        setError("Bitte gib deine E-Mail-Adresse oder deinen Username ein.");
+        return;
+      }
+
+      const result = isEmailInput
+        ? await authClient.signIn.email({
+            email: trimmedIdentifier,
+            password,
+          })
+        : await authClient.signIn.username({
+            username: trimmedIdentifier,
+            password,
+          });
 
       if (result.error) {
-        setError(result.error.message ?? "Anmeldung fehlgeschlagen");
+        setError(toGermanAuthError(result.error.message));
       } else {
         router.push("/dashboard");
       }
@@ -41,21 +93,28 @@ export default function SignInPage() {
     setError("");
     setMagicLinkMessage("");
 
-    if (!email) {
-      setError("Bitte gib deine E-Mail-Adresse ein");
+    const trimmedIdentifier = identifier.trim();
+
+    if (!trimmedIdentifier) {
+      setError("Bitte gib zuerst deine E-Mail-Adresse ein.");
+      return;
+    }
+
+    if (!trimmedIdentifier.includes("@")) {
+      setError("Magic Link funktioniert nur mit einer E-Mail-Adresse.");
       return;
     }
 
     setMagicLinkLoading(true);
     try {
       const result = await authClient.signIn.magicLink({
-        email,
+        email: trimmedIdentifier,
         callbackURL: "/dashboard",
         errorCallbackURL: "/auth/sign-in",
       });
 
       if (result.error) {
-        setError(result.error.message ?? "Magic Link konnte nicht gesendet werden");
+        setError(toGermanAuthError(result.error.message));
       } else {
         setMagicLinkMessage("Magic Link gesendet. Prüfe dein Postfach.");
       }
@@ -66,6 +125,12 @@ export default function SignInPage() {
     }
   };
 
+  const handleSocialPlaceholder = (provider: string) => {
+    setError("");
+    setMagicLinkMessage("");
+    setSocialMessage(`${provider}-Login ist aktuell als Platzhalter eingebunden.`);
+  };
+
   return (
     <div className="flex min-h-screen items-center justify-center bg-background">
       <div className="w-full max-w-sm space-y-6 rounded-xl border bg-card p-8 shadow-sm">
@@ -78,17 +143,19 @@ export default function SignInPage() {
 
         <form onSubmit={handleSignIn} className="space-y-4">
           <div>
-            <label htmlFor="email" className="block text-sm font-medium mb-1.5">
-              E-Mail
+            <label htmlFor="identifier" className="block text-sm font-medium mb-1.5">
+              E-Mail oder Username
             </label>
             <input
-              id="email"
-              type="email"
-              value={email}
-              onChange={(e) => setEmail(e.target.value)}
+              id="identifier"
+              type="text"
+              value={identifier}
+              onChange={(e) => setIdentifier(e.target.value)}
               required
               className="w-full rounded-lg border bg-background px-3 py-2 text-sm outline-none focus:ring-2 focus:ring-primary"
-              placeholder="name@beispiel.de"
+              placeholder="name@beispiel.de oder dein Username"
+              autoCapitalize="none"
+              autoCorrect="off"
             />
           </div>
 
@@ -111,6 +178,25 @@ export default function SignInPage() {
             <p className="text-sm text-red-500">{error}</p>
           )}
 
+          <div className="space-y-2">
+            <p className="text-xs text-center text-muted-foreground">Oder mit externen Anbietern</p>
+            {socialProviders.map((provider) => (
+              <button
+                key={provider.id}
+                type="button"
+                onClick={() => handleSocialPlaceholder(provider.name)}
+                className="w-full rounded-lg border bg-background px-4 py-2.5 text-sm font-medium hover:bg-muted transition-colors"
+              >
+                <span aria-hidden className="inline-block w-6 text-left">{provider.icon}</span>
+                {provider.name} {provider.subtitle}
+              </button>
+            ))}
+          </div>
+
+          {socialMessage && (
+            <p className="text-sm text-amber-600">{socialMessage}</p>
+          )}
+
           <button
             type="submit"
             disabled={loading}
diff --git a/app/auth/sign-up/page.tsx b/app/auth/sign-up/page.tsx
index f756ee7..60c131e 100644
--- a/app/auth/sign-up/page.tsx
+++ b/app/auth/sign-up/page.tsx
@@ -5,29 +5,171 @@ import { authClient } from "@/lib/auth-client";
 import { useRouter } from "next/navigation";
 import Link from "next/link";
 
+const socialProviders = [
+  {
+    id: "google",
+    name: "Google",
+    subtitle: "Platzhalter",
+    icon: "G",
+  },
+  {
+    id: "apple",
+    name: "Apple",
+    subtitle: "Platzhalter",
+    icon: "",
+  },
+];
+
+const MIN_USERNAME_LENGTH = 3;
+const MAX_USERNAME_LENGTH = 30;
+const MAX_USERNAME_ATTEMPTS = 8;
+
+function normalizeUsername(value: string) {
+  return value
+    .trim()
+    .toLowerCase()
+    .normalize("NFKD")
+    .replace(/[\u0300-\u036f]/g, "")
+    .replace(/[^a-z0-9._]+/g, ".")
+    .replace(/\.{2,}/g, ".")
+    .replace(/^[_\.]+|[_\.]+$/g, "");
+}
+
+function truncateWithSuffix(base: string, suffix = "") {
+  const allowedBaseLength = Math.max(MIN_USERNAME_LENGTH, MAX_USERNAME_LENGTH - suffix.length);
+  const safeBase = base.slice(0, allowedBaseLength);
+  return `${safeBase}${suffix}`.slice(0, MAX_USERNAME_LENGTH);
+}
+
+function fallbackUsernameFromInput(name: string, email: string) {
+  const emailLocalPart = email.split("@")[0] ?? "";
+  const fromName = normalizeUsername(name);
+  const fromEmail = normalizeUsername(emailLocalPart);
+  const candidate = fromName || fromEmail || "user";
+
+  if (candidate.length >= MIN_USERNAME_LENGTH) {
+    return candidate.slice(0, MAX_USERNAME_LENGTH);
+  }
+
+  return truncateWithSuffix(`${candidate}user`);
+}
+
+async function isUsernameAvailable(username: string) {
+  try {
+    const result = await authClient.isUsernameAvailable({ username });
+
+    if (result.error) {
+      return null;
+    }
+
+    return result.data?.available === true;
+  } catch {
+    return null;
+  }
+}
+
+async function getAvailableUsername(base: string) {
+  const normalizedBase = normalizeUsername(base);
+  const seeded =
+    normalizedBase.length >= MIN_USERNAME_LENGTH
+      ? normalizedBase
+      : truncateWithSuffix(`${normalizedBase || "user"}user`);
+
+  for (let attempt = 0; attempt < MAX_USERNAME_ATTEMPTS; attempt += 1) {
+    const suffix = attempt === 0 ? "" : `.${Math.floor(100 + Math.random() * 900)}`;
+    const candidate = truncateWithSuffix(seeded, suffix);
+    const available = await isUsernameAvailable(candidate);
+
+    if (available === true) {
+      return candidate;
+    }
+
+    if (available === null) {
+      return candidate;
+    }
+  }
+
+  return truncateWithSuffix(seeded, `.${Date.now().toString().slice(-4)}`);
+}
+
+function toGermanAuthError(message?: string) {
+  if (!message) {
+    return "Registrierung fehlgeschlagen. Bitte versuche es erneut.";
+  }
+
+  const normalized = message.toLowerCase();
+
+  if (normalized.includes("email") && normalized.includes("already")) {
+    return "Diese E-Mail-Adresse wird bereits verwendet.";
+  }
+
+  if (normalized.includes("username") && normalized.includes("already")) {
+    return "Dieser Username ist bereits vergeben.";
+  }
+
+  if (normalized.includes("password")) {
+    return "Das Passwort erfüllt die Anforderungen nicht.";
+  }
+
+  if (normalized.includes("invalid username")) {
+    return "Der Username enthält ungültige Zeichen.";
+  }
+
+  return "Registrierung fehlgeschlagen. Bitte prüfe deine Eingaben.";
+}
+
 export default function SignUpPage() {
   const router = useRouter();
   const [name, setName] = useState("");
   const [email, setEmail] = useState("");
+  const [username, setUsername] = useState("");
+  const [generateUsername, setGenerateUsername] = useState(true);
   const [password, setPassword] = useState("");
   const [error, setError] = useState("");
+  const [socialMessage, setSocialMessage] = useState("");
   const [success, setSuccess] = useState(false);
   const [loading, setLoading] = useState(false);
 
   const handleSignUp = async (e: React.FormEvent) => {
     e.preventDefault();
     setError("");
+    setSocialMessage("");
     setLoading(true);
 
     try {
+      const trimmedUsername = username.trim();
+      let finalUsername: string | undefined;
+
+      if (trimmedUsername) {
+        const normalizedInput = normalizeUsername(trimmedUsername);
+
+        if (normalizedInput.length < MIN_USERNAME_LENGTH) {
+          setError("Dein Username ist zu kurz (mindestens 3 Zeichen).");
+          return;
+        }
+
+        const availability = await isUsernameAvailable(normalizedInput);
+
+        if (availability === false) {
+          setError("Dieser Username ist bereits vergeben.");
+          return;
+        }
+
+        finalUsername = normalizedInput;
+      } else if (generateUsername) {
+        const generatedBase = fallbackUsernameFromInput(name, email);
+        finalUsername = await getAvailableUsername(generatedBase);
+      }
+
       const result = await authClient.signUp.email({
         email,
         password,
         name,
+        username: finalUsername,
       });
 
       if (result.error) {
-        setError(result.error.message ?? "Registrierung fehlgeschlagen");
+        setError(toGermanAuthError(result.error.message));
       } else {
         setSuccess(true);
       }
@@ -38,6 +180,11 @@ export default function SignUpPage() {
     }
   };
 
+  const handleSocialPlaceholder = (provider: string) => {
+    setError("");
+    setSocialMessage(`${provider}-Signup ist aktuell als Platzhalter eingebunden.`);
+  };
+
   if (success) {
     return (
       <div className="flex min-h-screen items-center justify-center bg-background">
@@ -102,6 +249,31 @@ export default function SignUpPage() {
             />
           </div>
 
+          <div className="space-y-2">
+            <label htmlFor="username" className="block text-sm font-medium mb-1.5">
+              Username (optional)
+            </label>
+            <input
+              id="username"
+              type="text"
+              value={username}
+              onChange={(e) => setUsername(e.target.value)}
+              className="w-full rounded-lg border bg-background px-3 py-2 text-sm outline-none focus:ring-2 focus:ring-primary"
+              placeholder="z. B. max.mustermann"
+              autoCapitalize="none"
+              autoCorrect="off"
+            />
+            <label className="flex items-center gap-2 text-xs text-muted-foreground">
+              <input
+                type="checkbox"
+                checked={generateUsername}
+                onChange={(e) => setGenerateUsername(e.target.checked)}
+                className="h-4 w-4 rounded border"
+              />
+              Username automatisch aus Name oder E-Mail generieren, wenn das Feld leer ist.
+            </label>
+          </div>
+
           <div>
             <label htmlFor="password" className="block text-sm font-medium mb-1.5">
               Passwort
@@ -122,6 +294,25 @@ export default function SignUpPage() {
             <p className="text-sm text-red-500">{error}</p>
           )}
 
+          <div className="space-y-2">
+            <p className="text-xs text-center text-muted-foreground">Oder mit externen Anbietern</p>
+            {socialProviders.map((provider) => (
+              <button
+                key={provider.id}
+                type="button"
+                onClick={() => handleSocialPlaceholder(provider.name)}
+                className="w-full rounded-lg border bg-background px-4 py-2.5 text-sm font-medium hover:bg-muted transition-colors"
+              >
+                <span aria-hidden className="inline-block w-6 text-left">{provider.icon}</span>
+                {provider.name} {provider.subtitle}
+              </button>
+            ))}
+          </div>
+
+          {socialMessage && (
+            <p className="text-sm text-amber-600">{socialMessage}</p>
+          )}
+
           <button
             type="submit"
             disabled={loading}
diff --git a/components/canvas/canvas.tsx b/components/canvas/canvas.tsx
index 95caf1c..7b27e73 100644
--- a/components/canvas/canvas.tsx
+++ b/components/canvas/canvas.tsx
@@ -333,6 +333,9 @@ function CanvasInner({ canvasId }: CanvasInnerProps) {
   const pendingMoveAfterCreateRef = useRef(
     new Map<string, { positionX: number; positionY: number }>(),
   );
+  const pendingResizeAfterCreateRef = useRef(
+    new Map<string, { width: number; height: number }>(),
+  );
   const resolvedRealIdByClientRequestRef = useRef(new Map<string, Id<"nodes">>());
   const pendingEdgeSplitByClientRequestRef = useRef(
     new Map<string, PendingEdgeSplit>(),
@@ -759,6 +762,7 @@ function CanvasInner({ canvasId }: CanvasInnerProps) {
     }
 
     pendingMoveAfterCreateRef.current.delete(args.clientRequestId);
+    pendingResizeAfterCreateRef.current.delete(args.clientRequestId);
     pendingEdgeSplitByClientRequestRef.current.delete(args.clientRequestId);
     pendingConnectionCreatesRef.current.delete(args.clientRequestId);
     resolvedRealIdByClientRequestRef.current.delete(args.clientRequestId);
@@ -1092,11 +1096,21 @@ function CanvasInner({ canvasId }: CanvasInnerProps) {
             });
           }
           if (op.type === "resizeNode" && process.env.NODE_ENV !== "production") {
+            const resizeNodeId =
+              typeof op.payload.nodeId === "string" ? op.payload.nodeId : null;
+            const resizeClientRequestId = resizeNodeId
+              ? clientRequestIdFromOptimisticNodeId(resizeNodeId)
+              : null;
+            const resizeResolvedRealId = resizeClientRequestId
+              ? resolvedRealIdByClientRequestRef.current.get(resizeClientRequestId)
+              : null;
             console.warn("[Canvas sync debug] resizeNode flush failed", {
               opId: op.id,
               attemptCount: op.attemptCount,
               transient,
               error: getErrorMessage(error),
+              clientRequestId: resizeClientRequestId,
+              resolvedRealId: resizeResolvedRealId ?? null,
               ...summarizeResizePayload(op.payload),
             });
           }
@@ -1259,13 +1273,67 @@ function CanvasInner({ canvasId }: CanvasInnerProps) {
     [enqueueSyncMutation],
   );
 
-  const runResizeNodeMutation = useCallback(
-    async (args: { nodeId: Id<"nodes">; width: number; height: number }) => {
-      await enqueueSyncMutation("resizeNode", args);
+  const flushPendingResizeForClientRequest = useCallback(
+    async (clientRequestId: string, realId: Id<"nodes">): Promise<void> => {
+      const pendingResize = pendingResizeAfterCreateRef.current.get(clientRequestId);
+      if (!pendingResize) return;
+      pendingResizeAfterCreateRef.current.delete(clientRequestId);
+      await enqueueSyncMutation("resizeNode", {
+        nodeId: realId,
+        width: pendingResize.width,
+        height: pendingResize.height,
+      });
     },
     [enqueueSyncMutation],
   );
 
+  const runResizeNodeMutation = useCallback(
+    async (args: { nodeId: Id<"nodes">; width: number; height: number }) => {
+      const rawNodeId = args.nodeId as string;
+      if (!isOptimisticNodeId(rawNodeId)) {
+        await enqueueSyncMutation("resizeNode", args);
+        return;
+      }
+
+      if (!isSyncOnline) {
+        await enqueueSyncMutation("resizeNode", args);
+        return;
+      }
+
+      const clientRequestId = clientRequestIdFromOptimisticNodeId(rawNodeId);
+      const resolvedRealId = clientRequestId
+        ? resolvedRealIdByClientRequestRef.current.get(clientRequestId)
+        : undefined;
+
+      if (resolvedRealId) {
+        await enqueueSyncMutation("resizeNode", {
+          nodeId: resolvedRealId,
+          width: args.width,
+          height: args.height,
+        });
+        return;
+      }
+
+      if (clientRequestId) {
+        pendingResizeAfterCreateRef.current.set(clientRequestId, {
+          width: args.width,
+          height: args.height,
+        });
+      }
+
+      if (process.env.NODE_ENV !== "production") {
+        console.info("[Canvas sync debug] deferred resize for optimistic node", {
+          nodeId: rawNodeId,
+          clientRequestId,
+          resolvedRealId: resolvedRealId ?? null,
+          width: args.width,
+          height: args.height,
+        });
+      }
+    },
+    [enqueueSyncMutation, isSyncOnline],
+  );
+
   const runUpdateNodeDataMutation = useCallback(
     async (args: { nodeId: Id<"nodes">; data: unknown }) => {
       await enqueueSyncMutation("updateData", args);
@@ -1521,6 +1589,7 @@ function CanvasInner({ canvasId }: CanvasInnerProps) {
             clientRequestId,
           );
           pendingMoveAfterCreateRef.current.delete(clientRequestId);
+          pendingResizeAfterCreateRef.current.delete(clientRequestId);
           pendingEdgeSplitByClientRequestRef.current.delete(clientRequestId);
           pendingConnectionCreatesRef.current.delete(clientRequestId);
           resolvedRealIdByClientRequestRef.current.delete(clientRequestId);
@@ -1572,6 +1641,7 @@ function CanvasInner({ canvasId }: CanvasInnerProps) {
               error: String(error),
             });
           }
+          await flushPendingResizeForClientRequest(clientRequestId, realId);
           return;
         }
 
@@ -1592,10 +1662,12 @@ function CanvasInner({ canvasId }: CanvasInnerProps) {
             positionX: pendingMove.positionX,
             positionY: pendingMove.positionY,
           });
+          await flushPendingResizeForClientRequest(clientRequestId, realId);
           return;
         }
 
         resolvedRealIdByClientRequestRef.current.set(clientRequestId, realId);
+        await flushPendingResizeForClientRequest(clientRequestId, realId);
         return;
       }
 
@@ -1643,6 +1715,7 @@ function CanvasInner({ canvasId }: CanvasInnerProps) {
     [
       canvasId,
       runBatchRemoveNodesMutation,
+      flushPendingResizeForClientRequest,
       runMoveNodeMutation,
       runSplitEdgeAtExistingNodeMutation,
     ],
diff --git a/convex/auth.ts b/convex/auth.ts
index 8e49e68..3c302de 100644
--- a/convex/auth.ts
+++ b/convex/auth.ts
@@ -8,7 +8,7 @@ import { internal } from "./_generated/api";
 import { DataModel } from "./_generated/dataModel";
 import { query } from "./_generated/server";
 import { betterAuth } from "better-auth/minimal";
-import { magicLink } from "better-auth/plugins";
+import { magicLink, username } from "better-auth/plugins";
 import { Resend } from "resend";
 import authConfig from "./auth.config";
 
@@ -128,10 +128,14 @@ export const createAuth = (ctx: GenericCtx<DataModel>) => {
           }
         },
       }),
+      username(),
       convex({ authConfig }),
       polar({
         client: polarClient,
-        createCustomerOnSignUp: true,
+        // Keep signup resilient: Polar customer sync is best-effort and should
+        // never fail account creation. Checkout/portal flows can still create
+        // and use customers via externalCustomerId when needed.
+        createCustomerOnSignUp: false,
         use: [
           checkout({
             successUrl: `${siteUrl}/dashboard?checkout=success`,
diff --git a/lib/auth-client.ts b/lib/auth-client.ts
index 0747a13..f9469af 100644
--- a/lib/auth-client.ts
+++ b/lib/auth-client.ts
@@ -1,9 +1,9 @@
 import { createAuthClient } from "better-auth/react";
-import { magicLinkClient } from "better-auth/client/plugins";
+import { magicLinkClient, usernameClient } from "better-auth/client/plugins";
 import { convexClient } from "@convex-dev/better-auth/client/plugins";
 import { polarClient } from "@polar-sh/better-auth/client";
 
 // Next.js: kein crossDomainClient nötig (same-origin via API Route Proxy)
 export const authClient = createAuthClient({
-  plugins: [magicLinkClient(), convexClient(), polarClient()],
+  plugins: [magicLinkClient(), usernameClient(), convexClient(), polarClient()],
 });