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.