yt-dlp for page links in every Link mode; app icon and Linux launcher entry

yt-dlp: a managed, checksum-verified, daily-updated yt-dlp in the app and
the dedicated server (--[no-]yt-dlp, --yt-dlp-dir). Pasted links are
probed: direct files stay links, pages are resolved for title, uploader,
duration and thumbnail. Everyone fetches it: mpv's ytdl hook on the
managed copy. I pass it on / the server fetches it: the best single-file
format, re-resolved once when its URL expires. Relay names never split
the /r/ path; viewers of server-fetched pages wait for the relay to go
live. No wire change.

Icon: two stars orbiting a play button (assets/icon/icon.svg, PNGs, the
Windows .ico). GTK app id moe.senkochan.watch_together; release builds
install their .desktop file and icons into ~/.local/share so Wayland
shows the icon; the window icon is set for X11. README shows the icon.

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

packages/server/lib/src/relay_routes.dart

@@ -34,11 +34,15 @@ class ActiveRelay {
           {RelayBuffer? buffer, required void Function() onLiveChanged}) =>
       ActiveRelay._(mediaId, name, sourceToken, buffer ?? RelayBuffer(), null, onLiveChanged);
 
-  /// The server reads [url] itself.
+  /// The server reads [url] itself: a media file as is, a web page
+  /// through [ytDlp]. [onFail] gets the reason when it cannot.
   static ActiveRelay fetch(int mediaId, String name, Uri url,
-      {RelayBuffer? buffer, required void Function() onLiveChanged}) {
+      {RelayBuffer? buffer,
+      YtDlp? ytDlp,
+      required void Function() onLiveChanged,
+      void Function(String reason)? onFail}) {
     final b = buffer ?? RelayBuffer();
-    final local = LocalRelayFeed(HttpRelayBytes(url), b);
+    final local = LocalRelayFeed(LinkRelayBytes(url, ytDlp: ytDlp), b);
     final r = ActiveRelay._(mediaId, name, null, b, local, onLiveChanged);
     unawaited(() async {
       try {
@@ -48,6 +52,7 @@ class ActiveRelay {
         onLiveChanged();
       } catch (e) {
         _log.warning('relay fetch: cannot read the link: $e');
+        if (!r._disposed) onFail?.call('$e');
       }
     }());
     return r;