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/playback/playback_controller.dart

@@ -16,6 +16,7 @@ import '../sync/state_diff.dart';
 import '../sync/user_seek.dart';
 import 'frame_watchdog.dart';
 import 'media_marks.dart';
+import 'yt_dlp_app.dart' show ytdlFormat;
 
 /// Owns one mpv [Player] (and its [VideoController]) per session: created
 /// by [init] on join, released by [dispose] on leave, swapped by
@@ -60,6 +61,26 @@ class PlaybackController {
   /// Called once when the watchdog sees a frozen picture.
   void Function()? onFrozen;
 
+  /// mpv's yt-dlp hook could not open the current link (its error text).
+  void Function(String error)? onYtdlError;
+
+  /// Points mpv's yt-dlp hook at [path] (the app's managed copy) for web
+  /// page links, at most 1080p. Once per player.
+  Future<void> useYtDlp(String path) async {
+    final p = _player;
+    if (p == null || identical(p, _ytdlSetOn)) return;
+    try {
+      final dynamic platform = p.platform;
+      await platform?.setProperty('script-opts', 'ytdl_hook-ytdl_path=$path');
+      await platform?.setProperty('ytdl-format', ytdlFormat);
+      _ytdlSetOn = p;
+    } catch (e) {
+      debugPrint('yt-dlp path not set: $e');
+    }
+  }
+
+  Player? _ytdlSetOn;
+
   /// Fingerprint of the picture on screen, installed by the video view
   /// (`ShownFrameProbe.sample`). Without it the watchdog stays idle.
   Future<int?> Function()? shownFrame;
@@ -298,6 +319,11 @@ class PlaybackController {
   // an error there while shaders are on is a shader that failed to load
   // or compile (mpv then renders without it).
   void _onLog(PlayerLog log) {
+    if (log.prefix == 'ytdl_hook' && (log.level == 'error' || log.level == 'fatal')) {
+      final text = log.text.trim().replaceFirst(RegExp(r'^ERROR:\s*'), '');
+      if (text.isNotEmpty) onYtdlError?.call(text);
+      return;
+    }
     if (log.prefix != 'libmpv_render') return;
     if (log.text.contains('suspected software renderer')) {
       _setRenderMode(RenderMode.softwareGl);

apps/desktop/lib/src/playback/yt_dlp_app.dart

@@ -0,0 +1,47 @@
+import 'dart:async';
+import 'dart:io';
+
+import 'package:path_provider/path_provider.dart';
+import 'package:sync_relay_core/core.dart';
+
+/// mpv's `ytdl-format` for links every player opens itself.
+const ytdlFormat = 'bv*[height<=1080]+ba/b';
+
+/// The app's yt-dlp (see [YtDlp]): in the app-support folder, shared with
+/// the hosted server. [probe] remembers what pasted links are for a while,
+/// so the Link preview and the swap ask once.
+class AppYtDlp {
+  static final instance = AppYtDlp();
+
+  /// [ytDlp]: a given one (tests); default in the app-support folder.
+  AppYtDlp([YtDlp? ytDlp]) : _ytDlp = ytDlp == null ? null : Future.value(ytDlp);
+
+  Future<YtDlp>? _ytDlp;
+  Future<YtDlp> get ytDlp => _ytDlp ??= () async {
+        final base = await getApplicationSupportDirectory();
+        return YtDlp(dir(base.path));
+      }();
+
+  static String dir(String appSupport) => '$appSupport${Platform.pathSeparator}yt-dlp';
+
+  /// Gets yt-dlp ready in the background (download on first run, a daily
+  /// update check afterwards).
+  void warmUp() => unawaited(ytDlp.then((y) => y.ensure()).catchError((_) => null));
+
+  final _probes = <String, (DateTime, Future<LinkProbe>)>{};
+  static const _probeTtl = Duration(minutes: 10);
+
+  Future<LinkProbe> probe(String url) {
+    final now = DateTime.now();
+    _probes.removeWhere((_, v) => now.difference(v.$1) > _probeTtl);
+    final hit = _probes[url];
+    if (hit != null) return hit.$2;
+    final f = ytDlp.then((y) => probeLink(Uri.parse(url), ytDlp: y));
+    _probes[url] = (now, f);
+    f.catchError((Object _) {
+      _probes.remove(url);
+      return LinkProbe.file(Uri(), -1);
+    });
+    return f;
+  }
+}