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/Main.qml

@@ -0,0 +1,16 @@
+import QtQuick
+
+QtObject {
+    id: shell
+
+    readonly property DesktopSurface desktop: DesktopSurface {}
+    readonly property TopBarSurface topBar: TopBarSurface {}
+    readonly property LauncherSurface launcher: LauncherSurface {}
+
+    Shortcut {
+        sequence: "Escape"
+        enabled: ShellState.launcherOpen
+        context: Qt.ApplicationShortcut
+        onActivated: ShellState.closeLauncher()
+    }
+}