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/shell/start_banner.dart

@@ -82,7 +82,8 @@ class StartCard extends StatelessWidget {
             child: Row(
               crossAxisAlignment: CrossAxisAlignment.end,
               children: [
-                if (!small) ...[
+                // A thumbnail already fills the card as the banner.
+                if (!small && !now.thumbnail) ...[
                   ArtCover(hash: now.posterHash, width: coverWidth),
                   const SizedBox(width: 24),
                 ],