Never leave the desktop without a wallpaper after a theme intro (#15000)

A theme switch that plays an intro gave the shell the new still only when
the intro failed. The shell kept the old theme's path, which the theme swap
deletes, and loaded it again when OWE handed the desktop back, leaving it
empty. The new still is now handed over either way.

The shell also re-reads the background link whenever OWE hands the desktop
back or a wallpaper fails to load, falling back to the theme's first
background when the link itself is broken.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

bin/omarchy-theme-bg-set

@@ -20,6 +20,6 @@ fi
 # Create symlink to the new background media
 ln -nsf "$BACKGROUND" "$CURRENT_BACKGROUND_LINK"
 
-# Update the live shell background immediately when it is running. The
-# background plugin also polls this symlink, but IPC avoids the visible delay.
+# Update the live shell background when it is running. Nothing watches the
+# symlink, so this is how the shell learns of the new background.
 omarchy-shell -q background set "$BACKGROUND"