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/session/connection.dart

@@ -391,7 +391,8 @@ class Connection {
   void sendReady(bool ready, {String source = 'manual'}) => client.sendReady(ready, source: source);
   void sendForce(String target, bool ready) => client.sendForce(target, ready);
   void sendSeek(double position, {int seq = 0}) => client.sendSeek(position, seq: seq);
-  void sendSwap(HelloMedia media, {String? queueId}) => client.sendSwap(media, queueId: queueId);
+  void sendSwap(HelloMedia media, {String? queueId, SwapRestore? restore}) =>
+      client.sendSwap(media, queueId: queueId, restore: restore);
   void sendQueue(QueueIntent intent) => client.sendQueue(intent);
   void sendReport({
     required double position,