## What happened

`omarchy launch webapp https://messenger.com` fails with:

```
error: path "--app=https://messenger.com" does not exist!
```

Expected: messenger.com opens as a web app (app mode) in the default browser.

## Reproduction

1. Install Firefox as the only browser and set it as default (`xdg-settings get default-web-browser` → `firefox.desktop`).
2. Run `omarchy launch webapp https://messenger.com`.

## Root cause

`/usr/share/omarchy/bin/omarchy-launch-webapp` whitelists only Chromium-family
browsers and falls back to `chromium.desktop` for anything else:

```bash
case $browser in
google-chrome* | brave* | microsoft-edge* | opera* | vivaldi* | helium*) ;;
*) browser="chromium.desktop" ;;
esac

exec setsid uwsm-app -- $(sed -n 's/^Exec=\([^ ]*\).*/\1/p' {~/.local,~/.nix-profile,/usr}/share/applications/$browser 2>/dev/null | head -1) --app="$1" "${@:2}"
```

With Firefox set as default, `chromium.desktop` doesn't exist, so the `sed`
extraction returns an empty binary path and the command degrades to:

```
uwsm-app -- --app="https://messenger.com"
```

`uwsm-app` then treats `--app=...` as the executable path → `error: path "... does not exist!"`.

Suggested fix: add Firefox to the supported browser list (Firefox supports
`--app=<url>`), and/or fall back to the actual default browser's desktop file
instead of hardcoding `chromium.desktop`.

## System details

- Omarchy: 4.0.0-1
- Arch x86_64, Intel Celeron N4020 (GeminiLake UHD 600)
- Default browser: `firefox.desktop`