fix(player): tell the user when the server cannot read the media file

A 404 on the media stream means the server resolved the item but could not
open the file behind it — moved, deleted, or on storage that went away.
Jellyfin maps the resulting FileNotFoundException to 404, and PlaybackInfo
never stats the file, so negotiation succeeds and only the stream request
fails. Playback then died with a snackbar reading "Failed to open
[REDACTED_URL]" before popping the route, which tells the user nothing and
leaves nothing useful in a bug report.

Generalize the HTTP-500 log probe into PlayerError.httpStatusFromLog and
latch every status in fatalPlaybackHttpStatuses. Each latches on its own so
the 503 that stream-lavf-o deliberately retries cannot mask the fatal status
behind it. A 404 now raises a dedicated modal naming the cause and the fix.

On Android a 404 previously failed the "Response code: 500" string test and
fell through to the ExoPlayer→MPV fallback, showing "switching to compatible
player" before failing again on the same request. Read the real status off
HttpDataSource.InvalidResponseCodeException instead and skip the fallback:
an HTTP status is not a codec problem.

lib/screens/video_player_screen.dart

@@ -1772,9 +1772,12 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
   }
 
   String? _lastLogError;
-  bool _sawServer500 = false;
 
-  static final RegExp _server500Pattern = RegExp(r'\b(?:HTTP error |Response code: )500\b');
+  /// Statuses in [fatalPlaybackHttpStatuses] the player's own log stream
+  /// reported for this open. Each latches independently: the reconnect path
+  /// deliberately retries a 503 (see `_applyNetworkStreamTuning`), and a
+  /// transient status must never mask the fatal one that follows.
+  final Set<int> _fatalHttpStatuses = <int>{};
 
   // OS Media Controls Integration