fix: cover the whole display while a disc loads

The loading surface was invisible. GtkWindow only renders its own CSS
background when the application is not painting the window itself, and
this window was created app-paintable with no draw handler and no child
— so it drew nothing at all. The black area during loading was never it:
it was VLC's own window, tiled by sway into the usable area, which
excludes the strip the dock reserves. That is why the dock and the
wallpaper stayed on screen underneath.

An earlier headless check missed this because that sway had no wallpaper
configured, so an invisible surface and a black one looked identical.
Rechecked with the real wallpaper and the real dock underneath.

The surface now paints, and carries a spinner and "Loading DVD..." on
the same translucent rounded card as the dock. It moves from the top
layer to the overlay layer, because the dock is a top-layer surface and
DVD mode has to cover it rather than depend on which surface sway
created first. Its exclusive zone of -1 keeps sway from moving it out of
the way of the dock's reserved strip.

It goes up before VLC is started rather than alongside it, so no part of
VLC's window is ever seen against the wallpaper, and comes down only
once VLC reports playback on screen. The deadline that takes it down
regardless is now twenty seconds, since an optical drive can be slow,
and a failure to start VLC at all ends the session instead of leaving a
cover with nothing behind it.

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

docs/testing.md

@@ -124,9 +124,12 @@ With `DVD_ENABLED=true`:
 
 ### DVD mode
 
-- [ ] Pressing DVD goes black immediately — no wallpaper, no dock, no
-      partial VLC window at any point
-- [ ] Video or the disc menu appears inside that black surface
+- [ ] Pressing DVD covers the **whole** display immediately, dock and
+      clock included — no wallpaper visible anywhere, at any point
+- [ ] A spinner and "Loading DVD…" show while the disc spins up
+- [ ] No partial VLC window is ever visible against the wallpaper
+- [ ] Video or the disc menu replaces the loading surface when ready
+- [ ] A disc that never opens still gives the desktop back (20s)
 - [ ] No VLC chrome anywhere: no menu bar, no fullscreen controller,
       no cone, no sway title bar
 - [ ] Wallpaper is never visible around VLC