feat: update dependencies and refactor layout and homepage components

- Added new dependencies: @daveyplate/better-auth-ui, next-themes, and sonner.
- Refactored layout component to use Providers and Toaster for better state management and notifications.
- Updated homepage to utilize authClient for session management and improved user experience with navigation links for sign-in and sign-up.

components/init-user.tsx

@@ -0,0 +1,33 @@
+"use client";
+
+import { authClient } from "@/lib/auth-client";
+import { useMutation, useQuery } from "convex/react";
+import { api } from "@/convex/_generated/api";
+import { useEffect } from "react";
+
+/**
+ * Initialisiert die Credit-Balance für neue User.
+ * Wird einmal im Layout eingebunden und sorgt dafür,
+ * dass jeder eingeloggte User eine Balance + Free-Subscription hat.
+ */
+export function InitUser() {
+  const { data: session } = authClient.useSession();
+  const balance = useQuery(
+    api.credits.getBalance,
+    session?.user ? {} : "skip"
+  );
+  const initBalance = useMutation(api.credits.initBalance);
+
+  useEffect(() => {
+    if (
+      session?.user &&
+      balance &&
+      balance.balance === 0 &&
+      balance.monthlyAllocation === 0
+    ) {
+      initBalance();
+    }
+  }, [session?.user, balance, initBalance]);
+
+  return null;
+}