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>

apps/desktop/lib/src/shell/library_tab.dart

@@ -83,7 +83,7 @@ class LibraryTab extends StatefulWidget {
   final VoidCallback? onOpenIntegrations;
   /// Defaults to [RecentLinks.instance].
   final RecentLinks? recentLinks;
-  final OEmbedGetter oembed;
+  final LinkProber probe;
   /// The Files source (tests inject one); defaults to [FilesPage].
   final Widget Function(BuildContext context, {required bool online, required bool relayed,
       required Future<String?> Function(String path) onPlay, String? nowPlayingPath})? filesBuilder;
@@ -100,7 +100,7 @@ class LibraryTab extends StatefulWidget {
     this.jellyfinStart,
     this.onOpenIntegrations,
     this.recentLinks,
-    this.oembed = defaultOEmbedGet,
+    this.probe = defaultLinkProber,
     this.filesBuilder,
     this.now = DateTime.now,
   });
@@ -245,7 +245,7 @@ class _LibraryTabState extends State<LibraryTab> {
           controller: _c,
           offlineHint: _hint,
           recents: widget.recentLinks ?? RecentLinks.instance,
-          oembed: widget.oembed,
+          probe: widget.probe,
           onPutUp: _put,
         ),
       MediaKind.file => _files(context),