feat: dress VLC down to an appliance screen

Stock VLC looks like a desktop media player, which is wrong for a room
where the TV is the whole interface. VLC stays the backend because its
dvdnav module is what provides the disc menus, and menus are the part
that matters on a commercial disc.

It is dressed down with documented VLC 3.x options, all verified against
the 3.0.23 build in Debian Trixie: minimal view, dark palette, no cone,
no tray, no now-playing popups, no resume prompt, no recent-disc
history, no title banner. sway draws no border on the window either, so
there is nothing above the video.

The auto-hiding fullscreen controller is deliberately kept: it is not
persistent chrome, and it is the only way a volunteer with a mouse can
pause or change chapter.

No skin is involved. Skins replace the interface wholesale and take the
keyboard and mouse handling that DVD menu navigation depends on with
them, as would --intf dummy.

Also drops --no-qt-updates-notif, which was not an option in Debian's
VLC at all. It made VLC exit before it started, so the DVD button could
never have launched a disc.

Co-Authored-By: Claude Opus 5 <[email protected]>

docs/testing.md

@@ -111,6 +111,9 @@ With `DVD_ENABLED=true`:
 - [ ] Closing the player works
 - [ ] Ejecting is detected, the icon goes back
 - [ ] Reinserting is detected again
+- [ ] VLC opens fullscreen, dark, with no menu bar, no orange cone and
+      no sway title bar
+- [ ] The fullscreen controller still appears on mouse movement
 - [ ] Commercial CSS disc, after `classroom-dvd-css-setup`
 
 ## OTA