fix: load login background video from local asset instead of network URL

VideoPlayerController.networkUrl(Uri.parse('assets/login-bg.mp4')) silently
fails because 'assets/login-bg.mp4' is not a valid HTTP URL — the video
never initializes and the login screen shows a plain black background.

Fix: switch to VideoPlayerController.asset() and register the file in
pubspec.yaml. The MP4 is gitignored (22 MB) and kept local for builds.

lib/screens/login_screen.dart

@@ -47,9 +47,7 @@ class _LoginScreenState extends State<LoginScreen> {
   }
 
   Future<void> _initVideo() async {
-    _videoController = VideoPlayerController.networkUrl(
-      Uri.parse('assets/login-bg.mp4'),
-    );
+    _videoController = VideoPlayerController.asset('assets/login-bg.mp4');
     await _videoController.initialize();
     _videoController.setLooping(true);
     _videoController.setVolume(0);