Fix admin login and CORS issues

- Update CORS configuration to allow frontend on port 3005
- Fix auth service to map email field to username for backend compatibility
- Add loading state handling in AdminLayout
- Add error boundary to dashboard route
- Fix unused parameter warning in login function

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/services/auth.service.ts

@@ -4,7 +4,11 @@ import type { LoginResponse, GalleryAuthResponse } from '../types';
 export const authService = {
   // Admin authentication
   async adminLogin(credentials: { email: string; password: string }): Promise<LoginResponse> {
-    const response = await api.post<LoginResponse>('/api/auth/admin/login', credentials);
+    // Backend expects 'username' field, but we accept email
+    const response = await api.post<LoginResponse>('/api/auth/admin/login', {
+      username: credentials.email,
+      password: credentials.password
+    });
     
     setAuthToken(response.data.token, true);
     return response.data;