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

.github/workflows/release-linux.yml

@@ -135,7 +135,7 @@ jobs:
           Terminal=false
           Type=Application
           Categories=AudioVideo;Audio;Music;Player;
-          Keywords=music;audio;qobuz;hifi;streaming;audiophile;
+          Keywords=music;audio;qobuz;hifi;streaming;audiophile;flac;
           StartupWMClass=com.blitzfc.qbz
           StartupNotify=true
           MimeType=x-scheme-handler/qbz;x-scheme-handler/qobuzapp;