## Summary

Large, high-resolution wallpapers look **grainy when set as the Omarchy background**, even though the same file looks clean when opened in an image viewer. The grain is aliasing caused by downscaling the wallpaper without mipmaps.

## Steps to reproduce

1. Set a large wallpaper (e.g. 6000x3274 PNG) with `omarchy theme bg set`.
2. Look at the desktop — dark/sky areas show visible fine grain.
3. Open the same file in an image viewer — it renders clean.

## Expected behavior

The background renders the same way as an image viewer: crisp, without added grain.

## Actual behavior

Visible noise/grain across the wallpaper, worst in darker areas.

## Root cause

In `/usr/share/omarchy/shell/plugins/background/Background.qml`, the main `base` `Image` (line 221) is missing `smooth: true` and `mipmap: true`, while the two transition frames (`oldFrame`/`incomingFrame`) already set both. Without mipmaps, a ~5.5x downscale (6000px -> 1080p) samples roughly one source pixel per screen pixel, aliasing fine texture into noise that reads as grain.

Measured on the affected image at 1080p: unfiltered downscale has 16.6 high-frequency stddev vs 8.5 with mipmap-quality (Lanczos) filtering.

## Proposed fix

```diff
       Image {
         id: base
         anchors.fill: parent
         source: root.imageUrl(root.displayedBackground)
         fillMode: Image.PreserveAspectCrop
         asynchronous: true
         cache: true
+        smooth: true
+        mipmap: true
         onStatusChanged: {
```

## System details

- Omarchy: 4.0.4-1
- GPU: Intel UHD (Alder Lake-P) + NVIDIA GeForce RTX 4050 Max-Q
- Displays: eDP-1 1920x1080@144 (scale 1.25), HDMI-A-1 1920x1080@144
- Affected wallpaper: 6000x3274 8-bit RGB PNG

## Visual comparison

`gh` cannot upload media, so attach these local files from the reproduction machine (or re-create them):

- `/tmp/opencode/before-after.png` — before (grainy, unfiltered) on the left, after (clean, mipmapped) on the right
- `/tmp/opencode/before-after-v.png` — same, stacked vertically