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_rows_test.dart

@@ -189,23 +189,21 @@ void main() {
       expect(decodeRecentLinks(jsonEncode([{'url': ''}, {'url': 'https://a/b.mkv'}])).single.title, 'b.mkv');
     });
 
-    test('preview: YouTube via oEmbed, other links by file name, failures fall back', () async {
-      Uri? asked;
-      final yt = await previewLink('https://www.youtube.com/watch?v=abc', get: (uri) async {
-        asked = uri;
-        return jsonEncode({'title': 'A video', 'author_name': 'Chan', 'thumbnail_url': 'https://i.ytimg/x.jpg'});
-      });
-      expect(asked!.host, 'www.youtube.com');
-      expect(asked!.path, '/oembed');
-      expect(asked!.queryParameters, {'url': 'https://www.youtube.com/watch?v=abc', 'format': 'json'});
-      expect((yt!.title, yt.subtitle, yt.thumbnail), ('A video', 'youtube.com · Chan', 'https://i.ytimg/x.jpg'));
+    test('preview: a page by yt-dlp, a file by its name, else why not', () async {
+      final yt = await previewLink('https://www.youtube.com/watch?v=abc',
+          probe: (url) async => LinkProbe.page(Uri.parse(url),
+              info: const YtDlpInfo(
+                  title: 'A video', uploader: 'Chan', duration: 187, thumbnail: 'https://i.ytimg/x.jpg')));
+      expect((yt!.title, yt.subtitle, yt.thumbnail), ('A video', 'youtube.com · Chan · 3:07', 'https://i.ytimg/x.jpg'));
 
       final file = await previewLink('https://cdn.example/dir/My%20Movie.mkv?sig=1',
-          get: (_) => throw StateError('no lookup for plain links'));
+          probe: (url) async => LinkProbe.file(Uri.parse(url), 10));
       expect((file!.title, file.subtitle, file.thumbnail), ('My Movie.mkv', 'cdn.example', null));
 
-      final down = await previewLink('https://youtu.be/abc', get: (_) async => throw Exception('offline'));
-      expect((down!.title, down.subtitle), ('abc', 'youtu.be'));
+      final page = await previewLink('https://example.com/a',
+          probe: (url) async => LinkProbe.page(Uri.parse(url), pageTitle: 'Example', ytDlpError: 'Unsupported URL'));
+      expect((page!.title, page.subtitle),
+          ('Example', "example.com · Not a video (and yt-dlp couldn't read it: Unsupported URL)"));
       expect(await previewLink('not a link'), isNull);
     });
 

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 {

apps/desktop/test/relay_e2e_test.dart

@@ -7,6 +7,7 @@ import 'package:sync_relay_core/core.dart';
 import 'package:sync_relay_server/server.dart';
 import 'package:watch_together/src/jellyfin/accounts.dart';
 import 'package:watch_together/src/jellyfin/track_prefs.dart';
+import 'package:watch_together/src/playback/yt_dlp_app.dart';
 import 'package:watch_together/src/screens/home/picked_media.dart';
 import 'package:watch_together/src/session/session_coordinator.dart';
 
@@ -67,7 +68,10 @@ void main() {
   Future<SessionCoordinator> member(String name) async {
     final acc = JellyfinAccounts(store: MemoryJellyfinStore());
     await acc.ensureLoaded(validate: false);
-    final c = SessionCoordinator(jellyfin: acc, trackPrefs: TrackPrefs(store: MemoryJellyfinStore()));
+    final c = SessionCoordinator(
+        jellyfin: acc,
+        trackPrefs: TrackPrefs(store: MemoryJellyfinStore()),
+        ytDlp: AppYtDlp(YtDlp('$_tmp/relay_e2e_no_ytdlp', enabled: false)));
     all.add(c);
     await c.connect(ConnectParams(host: '127.0.0.1', port: server.boundPort, secret: 'pw', name: name));
     await c.playback.player.setVolume(0);
@@ -150,7 +154,7 @@ void main() {
       await playTogether([alice, bob]);
       expect(hits.length, greaterThan(before));
 
-      // A page instead of a file is refused up front.
+      // A page yt-dlp cannot read is refused up front.
       final page = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
       page.listen((r) {
         r.response.headers.contentType = ContentType.html;
@@ -158,7 +162,7 @@ void main() {
         r.response.close();
       });
       expect(await alice.swapPicked(LinkMedia('http://127.0.0.1:${page.port}/watch?v=x', relay: LinkRelay.me)),
-          contains('cannot pass this link on'));
+          contains('Not a video'));
       await page.close(force: true);
     } finally {
       await origin.close(force: true);