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/surfaces/TopBarSurface.qml

@@ -0,0 +1,27 @@
+import QtQuick
+import QtQuick.Window
+import org.kde.layershell 1.0 as LayerShell
+import Nebula.UI
+
+Window {
+    id: root
+
+    title: qsTr("Nebula Top Bar")
+    color: "transparent"
+    flags: Qt.FramelessWindowHint | Qt.WindowDoesNotAcceptFocus
+    visible: true
+    width: Screen.width
+    height: Theme.topBarHeight
+
+    LayerShell.Window.scope: "nebula-topbar"
+    LayerShell.Window.layer: LayerShell.Window.LayerTop
+    LayerShell.Window.anchors: LayerShell.Window.AnchorTop
+        | LayerShell.Window.AnchorLeft
+        | LayerShell.Window.AnchorRight
+    LayerShell.Window.exclusionZone: Theme.topBarHeight
+    LayerShell.Window.keyboardInteractivity: LayerShell.Window.KeyboardInteractivityNone
+
+    TopBar {
+        anchors.fill: parent
+    }
+}