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/test/queue_switch_e2e_test.dart

@@ -14,6 +14,7 @@ import 'package:starsync/src/queue/queue_controller.dart' show WarmState;
 import 'package:starsync/src/screens/home/picked_media.dart';
 import 'package:starsync/src/session/session_coordinator.dart';
 import 'package:starsync/src/session/switch_trace.dart';
+import 'package:starsync/src/state/settings.dart';
 
 // How long an Up next switch takes through the media pipeline, end to
 // end, over real mpv and a real sync server: two Jellyfin account holders
@@ -244,8 +245,14 @@ void main() {
       final hosts = <SessionCoordinator, MediaKitHost>{};
       SessionCoordinator coord(JellyfinAccounts acc) {
         final host = MediaKitHost()..init();
+        // Up next alone: no continuation of the episodes (it would be
+        // queued after the items this test adds and times).
         final c = SessionCoordinator(
-            jellyfin: acc, trackPrefs: TrackPrefs(store: MemoryJellyfinStore()), ytDlp: ytDlp, players: host)
+            jellyfin: acc,
+            trackPrefs: TrackPrefs(store: MemoryJellyfinStore()),
+            ytDlp: ytDlp,
+            players: host,
+            settings: AppSettings()..autoAdvance.value = false)
           ..media.preloadEnabled = preload;
         hosts[c] = host;
         return c;