Swap shows only the new media, no guessed endings, quieter screen

- A swap stops the previous media at once; buffer counts only for the
  media the player holds (client) and the current mediaId (server), so
  the old video can no longer start the new one.
- The automatic frozen-picture reload is gone (false alarms on still
  scenes); Reload video stays.
- The "Next episode" pill shows only from a known ending.
- A web link's thumbnail is its cover, under the video's own name.
- While playing, the unread chip stays off the picture; chat pop-ups
  can be All / Only when paused / Off.
- Server fetch failures say when the server has no cookies.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/session/now_playing.dart

@@ -11,6 +11,10 @@ class NowPlaying {
   final String bannerHash;
   const NowPlaying({required this.title, this.episode, this.posterHash = '', this.bannerHash = ''});
 
+  /// The art is a web video's thumbnail (the same picture as cover and
+  /// banner): shown wide, under the video's own name.
+  bool get thumbnail => posterHash.isNotEmpty && posterHash == bannerHash;
+
   @override
   bool operator ==(Object other) =>
       other is NowPlaying &&