accept email on login

backend/src/main/java/com/petshop/backend/controller/AuthController.java

@@ -143,11 +143,16 @@ public class AuthController {
     @PostMapping("/login")
     public ResponseEntity<LoginResponse> login(@Valid @RequestBody LoginRequest request) {
         try {
+            String identifier = request.getUsername();
+            String resolvedUsername = identifier != null && identifier.contains("@")
+                    ? userRepository.findByEmail(identifier).map(User::getUsername).orElse(identifier)
+                    : identifier;
+
             authenticationManager.authenticate(
-                    new UsernamePasswordAuthenticationToken(request.getUsername(), request.getPassword())
+                    new UsernamePasswordAuthenticationToken(resolvedUsername, request.getPassword())
             );
 
-            User user = userRepository.findByUsername(request.getUsername())
+            User user = userRepository.findByUsername(resolvedUsername)
                     .orElseThrow(() -> new UsernameNotFoundException("User not found"));
 
             String token = jwtUtil.generateToken(user);