feat(scene): implement pause menu with UI manager and escape key toggling

Add PauseMenuUIManager class and integrate pause state into WorldScene.
Escape key toggles pause in ClientPlayer and PauseMenuUIManager.
Refactor Window mouse and imgui control to support pause state.

src/CMakeLists.txt

@@ -70,4 +70,5 @@ target_sources(${PROJECT_NAME}
         ui/column_layout.cpp
         ui/rect.cpp
         ui/ui_manager.cpp
+        ui/pause_menu_ui_manager.cpp
 )