Add Qt/QML desktop shell and Nebula.UI

Introduce a new production desktop shell at `shells/desktop/shell` built with C++20, Qt Quick/QML, and LayerShellQt, including top bar, launcher, desktop surfaces, shell state, and mock app data. Add the shared `packages/nebula-ui` QML module (theme, controls, focus, icons) and wire it into the shell build.

Retire the old GTK/WebKit native host by removing `shells/desktop/native`, move the React shell UI to `shells/desktop/prototype-react` as an archived design reference, and update root/compositor/session documentation to reflect the new Linux-only production architecture and temporary Sway role.

compositor/README.md

@@ -1,3 +1,27 @@
 # Compositor
 
-NebulaOS uses Sway as its Wayland compositor. Configuration lives in `sway/nebula-sway.conf`.
+Sway is the **temporary** Wayland compositor while Nebula Desktop is developed. Configuration lives in `sway/nebula-sway.conf`.
+
+Sway currently provides:
+
+- Wayland compositor
+- application window management
+- XWayland
+- input
+- output handling
+
+Nebula provides the visible shell (Qt Quick/QML via LayerShellQt). The Sway config must not enable a Sway bar.
+
+A custom Nebula compositor is a later milestone. Do not treat this Sway setup as the final architecture.
+
+```text
+Ubuntu/Linux
+ ↓
+Wayland
+ ↓
+Sway                 now
+Nebula compositor    later
+ ↓
+Nebula Shell
+Qt Quick/QML
+```