Add bilingual support (German/English) with language selector

- Added language selector in top-left corner
- Implemented data attributes for all translatable text
- Created language management system with localStorage persistence
- Updated all JavaScript functions to support both languages
- Added translations for notifications, error messages, and UI elements
- Maintained existing functionality while adding language switching

public/js/login.js

@@ -9,7 +9,11 @@ const supabase = window.supabase.createClient(SUPABASE_URL, SUPABASE_ANON_KEY);
 async function checkAuth() {
     const { data: { session } } = await supabase.auth.getSession();
     if (session) {
-        window.location.href = '/';
+        // Show a message that user is already logged in
+        showMessage('Sie sind bereits eingeloggt! Weiterleitung zum Dashboard...', 'success');
+        setTimeout(() => {
+            window.location.href = '/';
+        }, 2000);
     }
 }