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>

qml/SettingsPanel.qml

@@ -65,7 +65,7 @@ Flickable {
             SliderRow {
                 width: parent.width
                 text: qsTr("Background")
-                from: 0.1 // Settings::MinBackgroundOpacity
+                from: panel.settings.minBackgroundOpacity
                 value: panel.settings.backgroundOpacity
                 onMoved: value => panel.settings.backgroundOpacity = value
             }