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/linux/CMakeLists.txt

@@ -7,7 +7,7 @@ project(runner LANGUAGES CXX)
 set(BINARY_NAME "watch_together")
 # The unique GTK application identifier for this application. See:
 # https://wiki.gnome.org/HowDoI/ChooseApplicationID
-set(APPLICATION_ID "com.example.watch_together")
+set(APPLICATION_ID "moe.senkochan.watch_together")
 
 # Explicitly opt in to modern CMake behaviors to avoid warnings with recent
 # versions of CMake.

apps/desktop/linux/runner/my_application.cc

@@ -54,6 +54,19 @@ static void my_application_activate(GApplication* application) {
 
   gtk_window_set_default_size(window, 1280, 720);
 
+  // The window icon (X11; Wayland takes it from the .desktop file the app
+  // installs for its application id).
+  {
+    g_autofree gchar* exe = g_file_read_link("/proc/self/exe", nullptr);
+    if (exe != nullptr) {
+      g_autofree gchar* dir = g_path_get_dirname(exe);
+      g_autofree gchar* icon = g_build_filename(
+          dir, "data", "flutter_assets", "assets", "icon", "icon_256.png",
+          nullptr);
+      gtk_window_set_icon_from_file(window, icon, nullptr);
+    }
+  }
+
   g_autoptr(FlDartProject) project = fl_dart_project_new();
   fl_dart_project_set_dart_entrypoint_arguments(
       project, self->dart_entrypoint_arguments);