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.

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()],
 });