Keep the lock wallpaper decoded so waking shows it with the password field (#13431)

The lock started decoding its wallpaper only once locked, with the cache
off, and first at the view's unsized native resolution. A machine
suspending right after locking froze that decode partway, so waking
showed the password field on a bare background and the wallpaper
popped in after it. On this machine the wallpaper took ~208ms to become
ready, and the suspend followed the lock by 66ms.

The lock service now keeps each screen's lock wallpaper decoded in the
image cache, as the lock view requests it: same URL, the screen's
logical size, PreserveAspectCrop. The view waits for its size and reads
from the cache, so the wallpaper is ready within ~3ms of the lock
starting. The version in the cached URL follows the file's mtime and
size, so a wallpaper overwritten in place still reloads.

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

shell/plugins/lock/LockView.qml

@@ -96,6 +96,13 @@ Item {
       anchors.fill: parent
       path: root.loadBackground ? (root.video ? root.videoPosterPath : root.backgroundPath) : ""
       version: root.backgroundVersion
+      // Decode only once sized, at the lock's own size: an unsized first
+      // request decoded the file at its native resolution, then again once
+      // sized. That size is what the lock service keeps decoded ahead of the
+      // lock, so the first frame has the wallpaper.
+      cached: true
+      constrainDecode: true
+      decodeSize: Qt.size(width, height)
     }
 
     MultiEffect {