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/main.dart

@@ -66,8 +66,10 @@ Future<void> main(List<String> args) async {
   } catch (e, st) {
     debugPrint('load connect form failed: $e\n$st');
   }
-  // The library's matches, pins and offsets (device-owned).
+  // The library's matches, pins and offsets, and the watch history
+  // (device-owned).
   unawaited(graph.library.ensureLoaded());
+  unawaited(graph.watch.history.ensureLoaded());
   ports.players.init();
   thumbnailPlayers = ports.players;
   // yt-dlp for web page links: downloaded on first run, kept current.