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/test/library_tab_test.dart

@@ -108,10 +108,9 @@ Future<Harness> pumpLibrary(
         jellyfinStart: start,
         recentLinks: h.recents,
         now: () => fixtureNow,
-        oembed: (uri) async => jsonEncode({
-          'title': 'A video',
-          'author_name': 'Chan',
-        }),
+        probe: (url) async => url.endsWith('.mkv')
+            ? LinkProbe.file(Uri.parse(url), 1)
+            : LinkProbe.page(Uri.parse(url), info: const YtDlpInfo(title: 'A video', uploader: 'Chan')),
         filesBuilder: (context, {required online, required relayed, required onPlay, nowPlayingPath}) =>
             FilesStub(online: online, relayed: relayed, onPlay: onPlay),
         onPutUp: (m) async {