feat: register qbz:// URI scheme for OAuth callbacks

Add URI scheme handler registration for OAuth flow implementation:
- Register x-scheme-handler/qbz in .desktop files
- Add %u parameter to Exec for URL handling
- Add OpenURI portal permission in Flatpak manifest

This enables OAuth PKCE flows for Spotify and other providers
to redirect back to the application after browser authentication.

Files modified:
- packaging/arch/qbz.desktop
- packaging/aur/qbz.desktop
- packaging/flatpak/com.blitzkriegfc.qbz.desktop
- packaging/flatpak/com.blitzkriegfc.qbz.yml

packaging/aur/qbz.desktop

@@ -1,10 +1,11 @@
 [Desktop Entry]
 Name=QBZ
 Comment=Native Qobuz client with hi-res audio support
-Exec=qbz
+Exec=qbz %u
 Type=Application
 Icon=qbz
 Categories=Audio;Music;Player;AudioVideo;
 Terminal=false
 StartupWMClass=qbz-nix
 Keywords=music;audio;qobuz;hifi;streaming;
+MimeType=x-scheme-handler/qbz;