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/server_hub.dart

@@ -20,9 +20,11 @@ class ServerHub {
   final Map<String, RelicWebSocket> sockets = {};
   /// The server's Jellyfin login for the `/j/...` relay; null = no relay.
   JellyfinCredential? jellyfin;
+  /// Reads web pages for "the server fetches it"; null = files only.
+  final YtDlp? ytDlp;
 
   ServerHub(this.sessions, this.expectedSecret,
-      {SyncEngine? engine, this.hostKey = '', this.jellyfin})
+      {SyncEngine? engine, this.hostKey = '', this.jellyfin, this.ytDlp})
       : engine = engine ?? SyncEngine();
 
   /// The current media is a Jellyfin ref on the server [jellyfin] logs in to.
@@ -356,7 +358,9 @@ void mountSyncRoute(
                 late final ActiveRelay active;
                 active = relayReq.fetch
                     ? ActiveRelay.fetch(hub.engine.mediaId, name, link!,
-                        onLiveChanged: () => hub.relayLiveChanged(active))
+                        ytDlp: hub.ytDlp,
+                        onLiveChanged: () => hub.relayLiveChanged(active),
+                        onFail: (why) => wsErr(ws, "The server can't fetch this link: $why"))
                     : _memberRelay(hub, token, name);
                 hub.setRelay(active);
                 hub.changed();