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/utils/dialogs.dart

@@ -104,6 +104,28 @@ Future<void> showServerLimitDialog(BuildContext context) async {
   );
 }
 
+/// Shows the server-side 404 modal: the item exists but the server cannot read
+/// the file behind it, so nothing client-side can recover the playback.
+Future<void> showMediaUnreadableDialog(BuildContext context) async {
+  await showScopedDialog<void>(
+    context: context,
+    barrierDismissible: false,
+    builder: (ctx) => AlertDialog(
+      title: Text(t.messages.mediaUnreadableTitle),
+      content: Text(t.messages.mediaUnreadableBody),
+      actions: [
+        DialogActionButton(
+          autofocus: true,
+          onPressed: () => Navigator.of(ctx).pop(),
+          label: t.common.close,
+          isPrimary: true,
+          style: FilledButton.styleFrom(padding: _buttonPadding, shape: _buttonShape),
+        ),
+      ],
+    ),
+  );
+}
+
 /// Shows a delete confirmation dialog.
 /// Convenience wrapper around [showConfirmDialog] with destructive styling.
 Future<bool> showDeleteConfirmation(