### System details

Dell XPS 13 (Intel Core 5 320), Omarchy 4.0.4, quickshell 0.3.1, Qt 6.11.2

### What's wrong?

`omarchy-shell background setInstant <path>` blanks the desktop to black until the new image has decoded, so every instant switch to a large wallpaper flashes black. `background set` (the wiping transition) doesn't, because it keeps the old frame up until the new one is `Ready`.

In `shell/plugins/background/Background.qml`, the instant branch of `transitionBackground()` points `displayedBackground` straight at the new path. The `base` Image is `asynchronous: true`, so it drops the old picture and paints nothing until the new file decodes. Nothing sits behind it except the transparent panel, so the screen shows black for that time. For a 2560×1440 PNG of a few MB, that's a visible flash of 100 ms or more.

**Steps to reproduce**

1. Put two large PNGs (a few MB each) in a theme's `backgrounds/`.
2. Alternate between them with `omarchy-shell background setInstant /path/to/a.png` and `omarchy-shell background setInstant /path/to/b.png`. Calling it from a Hyprland `workspace.active` handler, to give each workspace its own wallpaper, shows the flash on every workspace switch.
3. The desktop goes black briefly on each switch. Running the same steps with `background set` gives no flash.

**Suggested fix**

Qt 6.8 added [`Image.retainWhileLoading`](https://doc.qt.io/qt-6/qml-qtquick-image.html#retainWhileLoading-prop), which keeps showing the current image until the new source is ready. One line on the `base` Image:

```qml
Image {
  id: base
  ...
  asynchronous: true
  cache: true
  retainWhileLoading: true
```

I checked this outside the shell with a minimal QML scene on Qt 6.11.2 (offscreen platform): an `asynchronous` Image swapping between two 2560×1440 PNGs, with frames grabbed while the second one loads.

- Without `retainWhileLoading`, every grabbed frame was pure black (mean brightness 0 in all 6 samples).
- With `retainWhileLoading: true`, the frame still showed the previous image (mean brightness 51, the old wallpaper).

The transition path should be unaffected: it already keeps `incomingBackground` visible until `base` reports `Ready`.