fix(deep-link): robust qobuzapp:// handler across all packages

- Auto-detect installed .desktop filename (com.blitzfc.qbz.desktop,
  qbz.desktop, qbz-nix.desktop) instead of hardcoding
- Run update-desktop-database after register/deregister so xdg-open
  and browsers pick up the MIME association immediately
- Fix v2_check_qobuzapp_handler to match all known desktop filenames
- Align StartupWMClass to com.blitzfc.qbz across all packaging formats
- Add StartupNotify=true to Arch/AUR/Snap desktop files

Fixes browser URI handler not working for .deb users (Discussion #44)

packaging/arch/qbz.desktop

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