Debounce settings writes and save them on SIGTERM

- DebouncedStore, taken out of WindowSettings, now also saves the opacity
  slider once a drag ends instead of on every step
- SIGTERM/SIGINT (how KWin stops the input method) quit normally, so
  pending geometry and settings are written
- The slider's minimum opacity comes from Settings instead of a copy in QML

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CMakeLists.txt

@@ -24,6 +24,7 @@ qt_add_library(sloposk_core STATIC
     layout/LayoutModel.h layout/LayoutModel.cpp
     window/VisibilityController.h window/VisibilityController.cpp
     window/WindowSettings.h window/WindowSettings.cpp
+    settings/DebouncedStore.h settings/DebouncedStore.cpp
     settings/Settings.h settings/Settings.cpp
 )
 
@@ -76,6 +77,7 @@ qt_add_executable(sloposk
     main.cpp
     app/DevHooks.h app/DevHooks.cpp
     app/DBusControl.h app/DBusControl.cpp
+    app/QuitOnSignal.h app/QuitOnSignal.cpp
     app/SystemLayouts.h app/SystemLayouts.cpp
     backend/InputMethodProtocol.h
     backend/KWinInputMethod.h backend/KWinInputMethod.cpp