fix: Use username field instead of display_name in profile

- Updated profile-actions.ts to use username column
- Updated ProfileForm.tsx to use username
- Updated settings page to pass username
- Matches database schema (profiles.username)

src/services/profile-actions.ts

@@ -20,13 +20,13 @@ export async function updateProfile(formData: FormData): Promise<ProfileUpdateRe
             return { success: false, error: 'Nicht autorisiert' };
         }
 
-        const displayName = formData.get('display_name') as string;
+        const username = formData.get('username') as string;
 
         const { error } = await supabase
             .from('profiles')
             .upsert({
                 id: user.id,
-                display_name: displayName?.trim() || null,
+                username: username?.trim() || null,
                 updated_at: new Date().toISOString(),
             });
 
@@ -98,14 +98,14 @@ export async function getProfile() {
 
         const { data: profile } = await supabase
             .from('profiles')
-            .select('id, display_name, avatar_url, created_at, updated_at')
+            .select('id, username, avatar_url, updated_at')
             .eq('id', user.id)
             .single();
 
         return {
             id: user.id,
             email: user.email,
-            display_name: profile?.display_name || null,
+            username: profile?.username || null,
             avatar_url: profile?.avatar_url || null,
             created_at: user.created_at,
         };