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/ROADMAP.md

@@ -14,9 +14,11 @@ off by deleting it in the same commit as the code. Wire changes also edit
 2. **Files**: next episode from Files (by parsed episode, with the
    countdown). Later: tracking for file series.
 
-3. **yt-dlp** (epic): players that open links themselves use mpv's ytdl
-   hook (bundled on Windows); then relay of single-file formats; then a
-   resolver for the Link preview (title, thumbnail, duration).
+3. **yt-dlp follow-ups** (see ARCHITECTURE "yt-dlp"): check that the
+   bundled Windows libmpv has the ytdl hook (Lua); if not, resolve in the
+   app and open the single-file format. YouTube: yt-dlp wants a JS runtime
+   (deno) for full extraction, and it only has split streams, so relaying
+   it would need muxing. Formats that need yt-dlp's cookies.
 
 4. **AniList account**: sign in (OAuth via a localhost callback), "My list"
    filter on the Schedule; today's "Following" (AnimeFlow's monitored