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/widgets/video_controls/sheets/video_settings_sheet.dart

@@ -730,17 +730,18 @@ class _VideoSettingsSheetState extends State<VideoSettingsSheet> {
           ),
 
         if (kDebugMode)
-          FocusableListTile(
-            leading: AppIcon(Symbols.bug_report_rounded, fill: 1, color: tokens(context).textMuted),
-            title: const Text('Simulate HTTP 500 from server'),
-            onTap: () {
-              final player = widget.player;
-              OverlaySheetController.of(context).close();
-              if (player is PlayerBase) {
-                player.debugSimulateServer500();
-              }
-            },
-          ),
+          for (final status in const [500, 404])
+            FocusableListTile(
+              leading: AppIcon(Symbols.bug_report_rounded, fill: 1, color: tokens(context).textMuted),
+              title: Text('Simulate HTTP $status from server'),
+              onTap: () {
+                final player = widget.player;
+                OverlaySheetController.of(context).close();
+                if (player is PlayerBase) {
+                  player.debugSimulateServerHttpError(status);
+                }
+              },
+            ),
       ],
     );
   }