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/queue/queue_logic.dart

@@ -1,66 +1,15 @@
 import 'package:sync_relay_core/core.dart';
 
-import '../jellyfin/session_media.dart' show compareMemberIds;
-import '../session/party.dart' show isPresent;
-
-// Who puts an Up next item up. Pure: only what every client sees in
-// `state` decides, so all clients agree without a wire message (like the
-// Jellyfin auto-advance swapper).
-
-/// Who puts [item] up: [id] a member id; null with [why] when nobody here
-/// can (the item is skipped), null without [why] when nobody is around
-/// right now (nothing happens).
-typedef QueuePerformer = ({String? id, String? why});
-
-/// - A link passed on by its adder, or the adder's file: only the adder
-///   (their app has the file, the cookies). Gone (disconnected, in its
-///   grace): it waits for them; left for good: skipped.
-/// - Jellyfin: the adder when it plays that server with its own account,
-///   else the lowest member id that does (`jf[serverId] == true`,
-///   present: `in` or joining). Nobody: skipped.
-/// - A link everyone opens, or one the server fetches: the adder when
-///   present, else the lowest present member id.
-QueuePerformer queuePerformer(QueueItem item, List<MemberView> members) {
-  MemberView? adder;
-  for (final m in members) {
-    if (m.id == item.by) adder = m;
-  }
-  bool active(MemberView m) => isPresent(m);
-  if (item.how.adderOnly) {
-    if (adder == null) return (id: null, why: '${_name(item)} left');
-    if (adder.presence == Presence.gone) return (id: null, why: null);
-    return (id: adder.id, why: null);
-  }
-  final ref = item.media.jellyfin;
-  if (item.how == QueueHow.jellyfin && ref != null) {
-    bool can(MemberView m) => active(m) && m.jf[ref.serverId] == true;
-    if (adder != null && can(adder)) return (id: adder.id, why: null);
-    final id = autoAdvanceLowest(members.where(can));
-    return id == null ? (id: null, why: 'nobody here has that Jellyfin server') : (id: id, why: null);
-  }
-  if (adder != null && active(adder)) return (id: adder.id, why: null);
-  return (id: autoAdvanceLowest(members.where(active)), why: null);
-}
-
-String _name(QueueItem item) => item.byName.isEmpty ? 'who added it' : item.byName;
-
-/// The lowest member id of [members] ("m2" before "m10"), or null.
-String? autoAdvanceLowest(Iterable<MemberView> members) {
-  final ids = [for (final m in members) m.id]..sort(compareMemberIds);
-  return ids.isEmpty ? null : ids.first;
-}
-
-/// Who takes an item nobody can put up off the queue (`remove` with why):
-/// the lowest member id that is connected (not gone).
-String? queueJanitor(List<MemberView> members) =>
-    autoAdvanceLowest(members.where((m) => m.presence != Presence.gone));
+// Up next's words for an item. Who puts an item up is core's
+// `queuePerformer` (whats_next.dart), read the same by the server and
+// every app.
 
 /// How [item] plays, in a few words for its row: "through Foxyz" (you:
 /// "through you"), "server fetches", "everyone fetches", "Jellyfin",
 /// "Foxyz's file".
 String queueHowLabel(QueueItem item, {required String selfId}) {
   final mine = item.by == selfId && selfId.isNotEmpty;
-  final who = mine ? 'you' : _name(item);
+  final who = mine ? 'you' : (item.byName.isEmpty ? 'who added it' : item.byName);
   return switch (item.how) {
     QueueHow.through => 'through $who',
     QueueHow.file => mine ? 'your file' : "$who's file",