Enhance authentication flow with username support and social login placeholders

- Updated sign-in and sign-up pages to allow users to log in with either email or username.
- Added social login options for Google and Apple, currently implemented as placeholders.
- Improved error handling with localized messages for authentication failures.
- Refactored input fields and validation logic to enhance user experience and accessibility.

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}