Desktop settings page opens slowly (~500ms) on fast hardware

### What Happened?

Clicking the Desktop plug in Switchboard takes about half a second before the UI appears. It feels like the page is building everything up front and doing blocking work (building all subpages, sync D-Bus calls, wallpaper scanning) before showing anything.

### Steps to Reproduce

- Open Settings
- Click the "Desktop" settings plug
- Observe the delay before the page renders

### Expected Behavior

Desktop settings should appear instantly (or close to it) on first open, without a noticeable pause.

I am not too familiar with the code, but a few hints could be to:

- Build subpages lazily: create/load each section when first selected instead of all at once.
- Make the AccountsService calls async/cached so the UI isn’t blocked by D-Bus on page load.
- Enumerate wallpapers in true async chunks and defer thumbnail work until after initial render.

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_