fix(packaging): Arch installs com.blitzfc.qbz.desktop for the MPRIS app icon

GNOME resolves the MPRIS media-widget app icon via DesktopEntry -> the installed
<id>.desktop -> Icon=. Rename the Arch desktop file to com.blitzfc.qbz.desktop,
set Icon=com.blitzfc.qbz, and install the hicolor icon as com.blitzfc.qbz.png so
the basename matches the published DesktopEntry. Other channels (AUR/Gentoo/Snap
+ Tauri bundle/release workflow) documented in qbz-nix-docs (release-time,
build-test required). Flatpak was already correct.

cc

packaging/arch/com.blitzfc.qbz.desktop

@@ -3,7 +3,7 @@ Name=QBZ
 Comment=Native Qobuz client with hi-res audio support
 Exec=qbz %u
 Type=Application
-Icon=qbz
+Icon=com.blitzfc.qbz
 Categories=AudioVideo;Audio;Music;Player;
 Terminal=false
 StartupWMClass=com.blitzfc.qbz