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.

shell/Ui/BackgroundVideo.qml

@@ -1,15 +1,15 @@
 import QtQuick
 import QtMultimedia
 
-// Deliberately a bare MediaPlayer and VideoOutput rather than the Video
-// convenience type: Video always builds an AudioOutput, and a muted sink still
-// decodes the audio stream and opens an audio client on every output.
+// The lock screen is the only user. Playback is always silent, so this is a
+// bare MediaPlayer and VideoOutput rather than the Video convenience type:
+// Video always builds an AudioOutput, and a muted sink still decodes the audio
+// stream and opens an audio client.
 Item {
   id: root
 
   property url mediaSource: ""
   property bool playbackEnabled: true
-  property bool audioEnabled: false
   property int mediaGeneration: 0
   property bool priming: false
   property int primingGeneration: -1
@@ -74,23 +74,12 @@ Item {
     fillMode: VideoOutput.PreserveAspectCrop
   }
 
-  // Sound is opted into per output: with a player per monitor, every output
-  // playing the track would layer copies of it. The sink is only built once
-  // the media reports a sound track, so a silent file never opens an audio
-  // client or its threads. Priming a paused player must not be heard.
-  Loader {
-    id: audioLoader
-    active: root.audioEnabled && player.hasAudio
-    sourceComponent: AudioOutput {
-      muted: root.priming || !root.playbackEnabled
-    }
-  }
-
+  // No audio output is built, so the lock never decodes an audio stream or
+  // opens an audio client.
   MediaPlayer {
     id: player
     source: root.mediaSource
     videoOutput: output
-    audioOutput: audioLoader.item
     loops: MediaPlayer.Infinite
     autoPlay: root.playbackEnabled
     onMediaStatusChanged: {