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/state/settings.dart

@@ -59,7 +59,9 @@ class AppSettings {
   final hwdecMode = signal(HwdecMode.auto);
   final hwdecCustom = signal('');
   final autoCheckUpdates = signal(true);
-  /// Jellyfin episodes: at the end, count down 10 s and put up the next.
+  /// An episode this app puts up (Jellyfin, a file of a local show) is
+  /// continued by the next one (Up next's `auto` item, the server's
+  /// countdown at its end).
   final autoAdvance = signal(true);
   /// Session: send ready=false when the video changes (a swap keeps the
   /// server's ready flags).