Replace the desktop video path with OWE

The desktop background no longer plays videos. OWE owns video
backgrounds, and the shell layer stays empty behind one. The shell keeps
stills, which OWE hands back to it.

Remove the desktop video pause plumbing that only existed to stop an
unseen player: the lock, idle, and battery service lookups, the
per-output fullscreen check, the first-screen audio opt-in, and the audio
output in BackgroundVideo. The lock screen keeps its own silent playback.

Update the background tests, the manual, and the package note.

manual/39-backgrounds.md

@@ -4,6 +4,6 @@ Every theme ships with its own set of backgrounds, and you can add extras of you
 
 You can do this most easily by going to _Install > Style > Background_ in the Omarchy Menu. That'll bring up the folder where the backgrounds for that theme is stored. Hit `Super + Shift + F` to start another file manager, find your background, copy it over.  Now it'll be included in the choices of backgrounds you can select between using `Super + Ctrl + Space`.
 
-Backgrounds can be videos as well as stills. Drop an `mp4`, `m4v`, `mov`, `webm`, `mkv`, or `avi` file in the same folder and it appears alongside the images, playing on a loop. Only your first monitor's wallpaper plays a video's sound track, through the default audio output at the system volume, and the lock screen stays silent. Playback stops on its own whenever nothing can see it — while a fullscreen window covers that monitor, while the screensaver is up, and once a locked screen has gone dark — but a video wallpaper still costs far more power than a still one, and each monitor decodes its own copy. When the OWE wallpaper engine is running, it takes over video backgrounds and the shell keeps only stills.
+Backgrounds can be videos as well as stills. Drop an `mp4`, `m4v`, `mov`, `webm`, `mkv`, or `avi` file in the same folder and it appears alongside the images. Videos are played by the OWE wallpaper engine. It decodes once for every monitor and keeps playback silent, and it stops playback whenever nothing can see it. The lock screen keeps its own silent playback. A video wallpaper still costs far more power than a still one.
 
 You can find a huge collection of cool curated backgrounds on https://github.com/dharmx/walls.