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

flatpak/com.blitzfc.qbz.desktop

@@ -7,5 +7,6 @@ Icon=com.blitzfc.qbz
 Categories=AudioVideo;Audio;Music;Player;
 Terminal=false
 StartupWMClass=com.blitzfc.qbz
-Keywords=music;audio;qobuz;hifi;streaming;audiophile;
+StartupNotify=true
+Keywords=music;audio;qobuz;hifi;streaming;audiophile;flac;
 MimeType=x-scheme-handler/qbz;x-scheme-handler/qobuzapp;