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>

packages/core/lib/src/sync_engine.dart

@@ -429,7 +429,11 @@ class SyncEngine {
     _setError(member, mediaId == this.mediaId ? error : '');
     _seen(member);
     if (jf != null) member.jf = Map.unmodifiable(jf);
-    member.buffered = member.out || !buffered.isFinite || buffered < 0 ? 0 : buffered;
+    // A report for an earlier media (sent before the swap reached the
+    // client) says nothing about this one's buffer.
+    final earlier = mediaId != 0 && mediaId != this.mediaId;
+    member.buffered =
+        member.out || earlier || !buffered.isFinite || buffered < 0 ? 0 : buffered;
     final m = media;
     // First report of the length wins; only for the media the client has open.
     if (m != null && m.duration <= 0 && mediaId == this.mediaId &&

packages/core/test/sync_engine_test.dart

@@ -429,6 +429,24 @@ void main() {
       expect(engine.members['b']!.inSet, isTrue);
       expect(engine.transport, Transport.held);
     });
+
+    test("the previous media's buffer never starts the new one", () {
+      join('a', 'alice');
+      join('b', 'bob');
+      engine.applyReady('a', true);
+      engine.applyReady('b', true);
+      engine.applySwap('a', ep2);
+      final oldEvent = engine.eventId - 1;
+      // Sent before the swap reached the clients, landing after it.
+      engine.applyReport('a', 30, 20.0, oldEvent, mediaId: 1);
+      engine.applyReport('b', 30, 20.0, oldEvent, mediaId: 1);
+      expect(engine.members['a']!.buffered, 0);
+      engine.tick();
+      expect(engine.transport, Transport.held);
+      engine.applyReport('a', 0, 6.0, engine.eventId, mediaId: 2);
+      engine.applyReport('b', 0, 6.0, engine.eventId, mediaId: 2);
+      expect(engine.transport, Transport.playing);
+    });
   });
 
   group('force with actor feed', () {

packages/server/lib/src/server_hub.dart

@@ -34,6 +34,23 @@ class ServerHub {
     return ref != null && cred != null && cred.serverId.isNotEmpty && cred.serverId == ref.serverId;
   }
 
+  /// What the swapper reads when the server cannot fetch a link ([why]
+  /// from the relay). A yt-dlp failure on a server without cookies says
+  /// so, unless cookies would not help (unknown site, gone, unreachable).
+  String fetchFailText(String why) {
+    const head = "The server can't fetch this link";
+    final y = ytDlp;
+    final plain = playErrorText(why);
+    final cookiesMightHelp = y != null &&
+        y.enabled &&
+        why.contains('yt-dlp') &&
+        !const {unsupportedUrlText, notFoundText, unreachableText}.contains(plain) &&
+        y.cookies.read(count: false) == null;
+    return cookiesMightHelp
+        ? '$head: it has no cookies (set them in Settings → Player → yt-dlp on the hosting app, or --yt-dlp-cookies)'
+        : '$head: $why';
+  }
+
   StateBroadcast stateBroadcast() => engine.broadcast(relay: relayAvailable);
 
   /// The relay of the current media (`/r/...`), if it is relayed.
@@ -362,7 +379,7 @@ void mountSyncRoute(
                     ? ActiveRelay.fetch(hub.engine.mediaId, name, link!,
                         ytDlp: hub.ytDlp,
                         onLiveChanged: () => hub.relayLiveChanged(active),
-                        onFail: (why) => wsErr(ws, "The server can't fetch this link: $why"))
+                        onFail: (why) => wsErr(ws, hub.fetchFailText(why)))
                     : _memberRelay(hub, token, name, media.relay!.tracks);
                 hub.setRelay(active);
                 hub.changed();