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>

docs/HOW-IT-WORKS.md

@@ -79,7 +79,9 @@ anyone except the host.
 
 ## Where video comes from
 
-- **Link**: you paste an http(s) URL. Every app plays it directly with mpv.
+- **Link**: you paste an http(s) URL. Every app plays it directly with mpv;
+  a web page (YouTube, X, ...) goes through yt-dlp, which the app and the
+  server download and update by themselves.
   A pasted Jellyfin link becomes the Jellyfin item below when possible.
 - **Jellyfin item**: picked in the app's library browser. Shared as "this
   item on this Jellyfin server", never as a link with someone's sign-in.