Add bakery.toml and packaging/arch to match bread ecosystem

Mirrors the build/distribution pattern used by the bread project:
- bakery.toml describes bos-settings as a bakery-managed package
- packaging/arch/PKGBUILD builds and installs the binary via cargo
- packaging/arch/bos-settings.desktop for app launchers
- LICENSE (MIT) required by PKGBUILD

packaging/arch/README.md

@@ -0,0 +1,25 @@
+Arch packaging
+==============
+
+`PKGBUILD` builds and installs `bos-settings` from source.
+
+## Local build
+
+```bash
+makepkg -si
+```
+
+## Before publishing to [breadway] repo
+
+1. Tag a release on GitHub.
+2. Update `pkgver` to match the tag.
+3. Update `source` to the release tarball URL.
+4. Run `updpkgsums` (or manually set `sha256sums`).
+
+## Runtime dependencies
+
+| Package | Required | Notes |
+|---------|----------|-------|
+| `gtk4` | yes | UI toolkit |
+| `glib2` | yes | always |
+| `snapper` | optional | snapshot management view |