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

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "QBZ",
-  "version": "1.1.20",
+  "version": "1.2.0",
   "identifier": "com.blitzfc.qbz",
   "build": {
     "beforeDevCommand": "npm run dev",