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/CMakeLists.txt

@@ -0,0 +1,41 @@
+cmake_minimum_required(VERSION 3.20)
+
+if(NOT TARGET Qt6::Quick)
+    project(NebulaUI LANGUAGES CXX)
+    find_package(Qt6 6.4 REQUIRED COMPONENTS Quick)
+    qt_standard_project_setup(REQUIRES 6.4)
+endif()
+
+qt_add_library(NebulaUI STATIC)
+
+set(NEBULA_UI_QML_DIR "${CMAKE_CURRENT_SOURCE_DIR}/qml/Nebula/UI")
+
+set(NEBULA_UI_QML_FILES
+    Theme.qml
+    NebulaButton.qml
+    NebulaCard.qml
+    NebulaIconButton.qml
+    NebulaTextField.qml
+    FocusFrame.qml
+    NebulaIcon.qml
+)
+
+set(NEBULA_UI_QML_PATHS)
+foreach(qml_file IN LISTS NEBULA_UI_QML_FILES)
+    set(qml_path "${NEBULA_UI_QML_DIR}/${qml_file}")
+    set_source_files_properties("${qml_path}" PROPERTIES
+        QT_RESOURCE_ALIAS "${qml_file}"
+    )
+    list(APPEND NEBULA_UI_QML_PATHS "${qml_path}")
+endforeach()
+
+set_source_files_properties("${NEBULA_UI_QML_DIR}/Theme.qml" PROPERTIES
+    QT_QML_SINGLETON_TYPE TRUE
+)
+
+qt_add_qml_module(NebulaUI
+    URI Nebula.UI
+    VERSION 1.0
+    RESOURCE_PREFIX /qt/qml
+    QML_FILES ${NEBULA_UI_QML_PATHS}
+)