Let OWE own video backgrounds while it runs

The background plugin now watches for the OWE daemon socket. While OWE is
running, the desktop yields video playback to it and the shell keeps
stills. The lock screen keeps its own playback.

This lets Omarchy cooperate with OWE without OWE editing shell.json, so
the engine can ship as a package.

Add a package-list note that owe-wallpaper-engine must be added once it is
packaged.

install/omarchy-base.packages

@@ -107,6 +107,8 @@ python-poetry-core
 ttfx
 qemu-user-static-binfmt
 qrencode
+# TODO: add owe-wallpaper-engine here once it is packaged. The shell yields
+# video backgrounds to it while its daemon runs.
 qt6-imageformats
 qt6-multimedia
 qt6-multimedia-ffmpeg