Swap shows only the new media, no guessed endings, quieter screen

- A swap stops the previous media at once; buffer counts only for the
  media the player holds (client) and the current mediaId (server), so
  the old video can no longer start the new one.
- The automatic frozen-picture reload is gone (false alarms on still
  scenes); Reload video stays.
- The "Next episode" pill shows only from a known ending.
- A web link's thumbnail is its cover, under the video's own name.
- While playing, the unread chip stays off the picture; chat pop-ups
  can be All / Only when paused / Off.
- Server fetch failures say when the server has no cookies.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/widgets/shown_frame_probe.dart

@@ -1,65 +0,0 @@
-import 'dart:ui' as ui;
-
-import 'package:flutter/rendering.dart';
-import 'package:flutter/widgets.dart';
-
-import '../playback/frame_watchdog.dart';
-
-/// Fingerprints what the compositor really shows of the video. Place
-/// [anchor] under a `RepaintBoundary` around the video (the session player
-/// does); [sample] captures that boundary around the newest mounted anchor
-/// at ~200 px wide — the texture included — and hashes its centre. This is the only per-presented-frame signal: mpv's own
-/// counters keep ticking when the Flutter texture is stuck.
-class ShownFrameProbe {
-  final _anchors = <_AnchorState>[];
-
-  Widget anchor() => _Anchor(probe: this);
-
-  Future<int?> sample() async {
-    for (final a in _anchors.reversed) {
-      if (!a.mounted) continue;
-      final boundary = a.context.findAncestorRenderObjectOfType<RenderRepaintBoundary>();
-      if (boundary == null || !boundary.attached || !boundary.hasSize) return null;
-      final width = boundary.size.width;
-      if (width <= 0) return null;
-      ui.Image? image;
-      try {
-        image = await boundary.toImage(pixelRatio: (200 / width).clamp(0.02, 1.0));
-        final data = await image.toByteData(format: ui.ImageByteFormat.rawRgba);
-        if (data == null) return null;
-        return hashCenterPixels(data, image.width, image.height);
-      } catch (_) {
-        // Route not painted (covered) or mid-teardown: no reading.
-        return null;
-      } finally {
-        image?.dispose();
-      }
-    }
-    return null;
-  }
-}
-
-class _Anchor extends StatefulWidget {
-  final ShownFrameProbe probe;
-  const _Anchor({required this.probe});
-
-  @override
-  State<_Anchor> createState() => _AnchorState();
-}
-
-class _AnchorState extends State<_Anchor> {
-  @override
-  void initState() {
-    super.initState();
-    widget.probe._anchors.add(this);
-  }
-
-  @override
-  void dispose() {
-    widget.probe._anchors.remove(this);
-    super.dispose();
-  }
-
-  @override
-  Widget build(BuildContext context) => const SizedBox.shrink();
-}