Restructure step 6: what's next and watch tracking

- What's next is the server's: state.advance counts down (10 s) when the
  media finishes with an Up next item first, holds while its performer or
  warm source is gone, and sets queuePlay at its end; cancel is shared, a
  seek or swap ends it; the server skips items nobody can put up. Shared
  rules in core whats_next.dart; the app's janitor, per-screen countdown
  and end-of-episode card are gone.
- The continuation (next Jellyfin episode, next file of a local show
  with its gaps) is an auto item under "Continues with", prepared like
  Up next's first item.
- Watch tracking by time In (D3): only time played while this viewer is
  in counts; away, paused, held, jumps and stalls never do (P7, P9).
  WatchTracker + device WatchHistory (files, links); Jellyfin progress
  reads the same watch; records are the AniList seam.
- A server restart no longer loses what played: apps hold their media
  and one offers it back (swap.restore, first offer wins), Up next is
  re-added, the source re-feeds, and the watch carries on.

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

apps/desktop/lib/src/shell/session_player.dart

@@ -419,16 +419,14 @@ class _SessionPlayerState extends State<SessionPlayer> {
     });
   }
 
-  /// The action area's right end: the end-of-media card (Up next, else the
-  /// next episode; it stays up), else the pills, shown only with the
+  /// The action area's right end: the end-of-media card (Up next, else
+  /// the continuation; it stays up), else the pills, shown only with the
   /// controls ([PillGate]).
   Widget _actionSide() {
     final coord = _coord;
     return SignalBuilder(
       builder: (context) {
         final next = coord.nextEpisode.value;
-        final countdown = coord.autoAdvance.value;
-        // Up next goes before the next episode.
         final upNext = coord.queue.countdown.value;
         final finished = coord.serverState.value?.transport == 'finished';
         final pill = coord.nextPromptVisible.value;
@@ -437,18 +435,6 @@ class _SessionPlayerState extends State<SessionPlayer> {
         if (upNext != null && finished) {
           return ControlsHoldRegion(hold: _hold, child: UpNextCard(queue: coord.queue, countdown: upNext));
         }
-        // "Clear" in Up next hides the next episode for this media.
-        if (next != null && (finished || countdown != null) && coord.queue.shownContinuation.value.isNotEmpty) {
-          return ControlsHoldRegion(
-            hold: _hold,
-            child: EndOfEpisodeCard(
-              next: next,
-              countdown: countdown,
-              onPlayNow: coord.playNextEpisode,
-              onCancel: coord.cancelAutoAdvance,
-            ),
-          );
-        }
         if (!showNext && skip == null) return const SizedBox.shrink();
         return Column(
           mainAxisSize: MainAxisSize.min,