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.

packages/nebula-ui/qml/Nebula/UI/NebulaCard.qml

@@ -0,0 +1,21 @@
+import QtQuick
+
+Rectangle {
+    id: root
+
+    color: Theme.surface
+    border.color: Theme.border
+    border.width: 1
+    radius: Theme.radiusLarge
+
+    Rectangle {
+        id: shadow
+        z: -1
+        anchors.fill: parent
+        anchors.topMargin: 10
+        anchors.bottomMargin: -10
+        radius: root.radius
+        color: Theme.shadow
+        opacity: 0.85
+    }
+}