Adopt Voxtype upstream OSD theming after release

[repparw]

Context

Keep the upstream Voxtype Quickshell OSD for now. The local QML override was removed because it did not render reliably.

Voxtype's declarative OSD theming support is upstreamed in PR #501, but the release/Nixpkgs revision currently used here does not include it.

Follow-up

• Watch for a Voxtype release containing #501.
• Confirm that release is available through the Nixpkgs pin used by this flake.
• Configure the upstream style, palette, and layout options to match the desired OSD.
• Rebuild and verify that the upstream OSD still renders correctly.
• Close this issue with the Voxtype release and Nixpkgs revision that supplied the theming support.

Blocker: upstream release and subsequent Nixpkgs availability.