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/FocusFrame.qml

@@ -0,0 +1,24 @@
+import QtQuick
+
+Item {
+    id: root
+
+    property Item target: parent
+    property real radius: Theme.radiusSmall
+    property bool active: target && target.activeFocus
+
+    anchors.fill: target
+    anchors.margins: -2
+    z: 20
+    enabled: false
+    visible: active
+    Accessible.ignored: true
+
+    Rectangle {
+        anchors.fill: parent
+        color: "transparent"
+        border.width: 2
+        border.color: Theme.focus
+        radius: root.radius + 2
+    }
+}