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/main.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:io';
 
 import 'package:flutter/material.dart';
@@ -7,6 +8,8 @@ import 'package:sync_relay_core/core.dart';
 import 'package:window_manager/window_manager.dart';
 
 import 'app.dart';
+import 'src/playback/yt_dlp_app.dart';
+import 'src/platform/desktop_entry.dart';
 import 'src/screens/home/ui.dart';
 import 'src/state/session_state.dart';
 import 'src/state/settings.dart';
@@ -60,6 +63,10 @@ Future<void> main(List<String> args) async {
     debugPrint('load connect form failed: $e\n$st');
   }
   MediaKit.ensureInitialized();
+  // yt-dlp for web page links: downloaded on first run, kept current.
+  AppYtDlp.instance.warmUp();
+  // Linux: the launcher entry and icon Wayland needs (release installs).
+  unawaited(installDesktopEntry());
   runApp(const WatchTogetherApp());
 }