### System details

- Omarchy 4.0.0-1
- AMD Ryzen 5 7600X
- NVIDIA GeForce RTX 4070 and AMD Raphael integrated graphics

### What's wrong?

`omarchy-webapp-install` accepts any downloaded `image/*`, but saves every downloaded icon to a filename ending in `.png` without converting its contents. When a site serves WebP, the launcher can therefore receive a `.png` file containing WebP data and show no app icon.

### Steps to reproduce

Run:

```bash
omarchy webapp install Threads https://www.threads.com/ ""
file ~/.local/share/icons/hicolor/256x256/apps/threads.png
grep '^Icon=' ~/.local/share/applications/Threads.desktop
```

Observed on a clean temporary `$HOME`:

```text
threads.png: RIFF (little-endian) data, WebP image, with alpha, 57x57
Icon=threads
```

The Threads entry is created, but its icon is missing in the app launcher.

### Expected behavior

Downloaded icons stored with a `.png` filename should contain PNG data so icon-theme consumers can render them reliably.

### Relevant code

`download_icon()` validates only that the payload MIME type matches `image/*`. Both the automatic site-icon path and explicit icon-URL path pass a destination ending in `.png`, but neither converts the downloaded payload.

Since ImageMagick is part of the Omarchy base package set, the downloaded image could be validated in a temporary file and encoded as a real PNG at the destination.