chore(packaging): unify .desktop files across all formats

- Add flac; keyword to all .desktop files (was only in deb template)
- Add StartupNotify=true to flatpak .desktop files (was missing)
- Add Keywords line to snap .desktop (was missing entirely)
- Unify shortDescription to match other .desktop Comment fields

cc

src-tauri/tauri.conf.json

@@ -41,7 +41,7 @@
       "icons/icon.ico"
     ],
     "category": "Music",
-    "shortDescription": "Native high-fidelity client for Qobuz\u2122 subscribers on Linux",
+    "shortDescription": "Native Qobuz client with hi-res audio support",
     "longDescription": "QBZ is a native high-fidelity client for Qobuz\u2122 subscribers on Linux, designed for audiophiles who need bit-perfect playback without browser sample rate limitations. This application uses the Qobuz API but is not certified by Qobuz.",
     "linux": {
       "deb": {