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/controls/minimal/minimal_controls.dart

@@ -148,7 +148,7 @@ class MinimalControls extends StatelessWidget {
         const Expanded(
           child: Align(alignment: AlignmentDirectional.centerStart, child: NowPlayingTitle()),
         ),
-        UpNextButton(queue: m.queue, onPlayNextEpisode: m.playNextEpisode, look: UpNextLook.minimal, hold: hold),
+        UpNextButton(queue: m.queue, look: UpNextLook.minimal, hold: hold),
         const SizedBox(width: 4),
         SignalBuilder(
           builder: (context) {

apps/desktop/lib/src/controls/model/controls_model.dart

@@ -81,13 +81,16 @@ class PlayerControlsModel {
   /// Everything the layout depends on (not the position: that changes
   /// four times a second and only the timeline needs it).
   late final Computed<ControlsSnapshot> snapshot = computed(() {
-    final items = queue.items.value;
+    final queued = queue.items.value.firstOrNull;
+    final continues = queue.continues.value;
     final next = coord.nextEpisode.value;
     return ControlsSnapshot(
       play: play.value,
       previousLabel: coord.previousEpisode.value?.label,
-      nextLabel: items.isNotEmpty ? items.first.title : next?.label,
-      nextIsUpNext: items.isNotEmpty,
+      // Up next's first item, else the next episode, else what continues
+      // this (a file's next episode).
+      nextLabel: queued?.title ?? next?.label ?? continues?.title,
+      nextIsUpNext: queued != null,
       hasChapters: chapters.value.isNotEmpty,
       hasDuration: duration.value > Duration.zero,
       alone: coord.alone.value,
@@ -137,9 +140,10 @@ class PlayerControlsModel {
 
   void pickChapter(MediaChapter c) => coord.seekTo(c.start.inMicroseconds / 1e6);
 
-  /// Up next's first item, else the next episode. False when neither.
+  /// What plays next (Up next's first item, else the continuation), else
+  /// the next episode. False when neither.
   bool next() {
-    if (queue.items.peek().isNotEmpty) {
+    if (queue.head.peek() != null) {
       queue.skip();
       return true;
     }

apps/desktop/lib/src/controls/standard/standard_controls.dart

@@ -165,7 +165,7 @@ class StandardControls extends StatelessWidget {
     return Row(
       children: [
         const Expanded(child: NowPlayingTitle()),
-        UpNextButton(queue: m.queue, onPlayNextEpisode: m.playNextEpisode, look: UpNextLook.standard, hold: hold),
+        UpNextButton(queue: m.queue, look: UpNextLook.standard, hold: hold),
         SignalBuilder(
           builder: (context) {
             final full = m.fullscreen.value;