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.

.gitignore

@@ -50,3 +50,5 @@ web/assets/login-bg.mp4
 # Keystore files (signing keys)
 *.jks
 *.keystore
+# large binary assets — keep local only, not tracked in git
+assets/login-bg.mp4