feat: standardize login response format with code, message, and data wrapper

Update login endpoint to return consistent API response structure with code, message, and data fields. Wrap token, refreshToken, and user data inside a data object to match the standardized response format used across other endpoints.

Backend/src/services/auth.service.ts

@@ -58,9 +58,13 @@ export class AuthService {
         logger.info('User logged in successfully', { userId: user.id, email: user.email });
 
         return {
-            token,
-            refreshToken,
-            user: await this.formatUserResponse(user)
+            code: 200,
+            message: 'Login successful',
+            data: {
+                token,
+                refreshToken,
+                user: await this.formatUserResponse(user)
+            }
         };
     }