add ModalPauseScope for the pause-around-modal idiom

Four MainWindow sites hand-rolled snapshot speed / setGameSpeed(0) / modal /
restore + resetFrameTimer, with the restore duplicated on early-return paths.
ModalPauseScope does it via RAII, with restore()/release() for the two sites
that must restore early or not at all.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GH8ZMRY3vhxxXcaUxBqxkk

src/ui/CMakeLists.txt

@@ -4,6 +4,7 @@ SET(HDRS
     ${CMAKE_CURRENT_SOURCE_DIR}/VisualsLoader.h
     ${CMAKE_CURRENT_SOURCE_DIR}/MainWindow.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ModalDimOverlay.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/ModalPauseScope.h
     ${CMAKE_CURRENT_SOURCE_DIR}/GameWorldView.h
     ${CMAKE_CURRENT_SOURCE_DIR}/HeaderBar.h
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildButtonGrid.h