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.