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.

shells/desktop/shell/qml/ShellState.qml

@@ -0,0 +1,25 @@
+pragma Singleton
+
+import QtQuick
+
+QtObject {
+    property bool launcherOpen: false
+    property string activeApplication: "Desktop"
+
+    function openLauncher() {
+        launcherOpen = true
+    }
+
+    function closeLauncher() {
+        launcherOpen = false
+    }
+
+    function toggleLauncher() {
+        launcherOpen = !launcherOpen
+    }
+
+    function launch(name) {
+        activeApplication = name
+        closeLauncher()
+    }
+}