Display user's name instead of email in profile button

- Add name field to auth login/register responses
- Update User interface in AuthContext to include name
- Show name (or email as fallback) in navbar dropdown

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/context/AuthContext.tsx

@@ -10,6 +10,7 @@ import { authApi } from '../api/client';
 interface User {
   id: number;
   email: string;
+  name: string | null;
 }
 
 interface AuthContextType {