chore: bump version to 1.2.0

Updated:
- src-tauri/Cargo.toml
- src-tauri/tauri.conf.json
- package.json + package-lock.json
- snapcraft.yaml (root + packaging/)
- flake.nix
- Cargo.lock
- metainfo.xml (new release entry)
- README: Snap MPRIS limitation note with forum link

cc

package.json

@@ -1,6 +1,6 @@
 {
   "name": "qbz-nix",
-  "version": "1.1.20",
+  "version": "1.2.0",
   "description": "Native high-fidelity client for Qobuz™ subscribers on Linux with Hi-Fi audio support (uses the Qobuz API; not certified by Qobuz)",
   "type": "module",
   "scripts": {