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>

shell/Ui/BackgroundMedia.qml

@@ -18,6 +18,8 @@ Item {
   property size decodeSize: Qt.size(0, 0)
   readonly property var current: imageLoader.item
   readonly property bool ready: current ? current.ready : false
+  // The file behind path could not be loaded, as when it was deleted.
+  readonly property bool failed: current ? current.failed : false
   readonly property bool video: Util.isVideoPath(path)
   // Cache-bust images selected in a running lock session, so a theme switch
   // that replaces the file behind an unchanged path shows the new pixels.
@@ -35,6 +37,7 @@ Item {
 
     Image {
       readonly property bool ready: status === Image.Ready
+      readonly property bool failed: status === Image.Error
       source: !root.constrainDecode || root.decodeSize.width > 0 ? root.imageUrl : ""
       fillMode: Image.PreserveAspectCrop
       asynchronous: true