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/party.dart

@@ -12,8 +12,8 @@ const presencePolicy = PresencePolicy();
 bool countsForReady(MemberView m) => m.error.isEmpty && presencePolicy.countsFor(m.presence, Purpose.readyCounter);
 
 /// Connected and watching (`in` or `joining`): can do things for the
-/// party now (put up an Up next item, the auto-advance swap).
-bool isPresent(MemberView m) => m.presence == Presence.watching || m.presence == Presence.joining;
+/// party now (put up an Up next item; core's [isPresentMember]).
+bool isPresent(MemberView m) => isPresentMember(m);
 
 /// Nobody but [selfId] is connected in [members] (D9, core's [isAlone]).
 bool aloneIn(List<MemberView> members, String selfId) =>