See-through keyboard background with optional blur

- Opacity slider for the panel behind the keys (10–100%); keys stay opaque
- Blur Behind: asks the compositor (KWin) to blur under the rounded panel
  via KF6 WindowSystem; skipped at full opacity
- Settings panel draws over the window background so changes show live,
  and scrolls when it doesn't fit

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

src/CMakeLists.txt

@@ -39,6 +39,7 @@ set(SLOPOSK_QML_FILES
     SettingsButton.qml
     SettingsPanel.qml
     ToggleRow.qml
+    SliderRow.qml
     Theme.qml
 )
 list(TRANSFORM SLOPOSK_QML_FILES PREPEND ${PROJECT_SOURCE_DIR}/qml/ OUTPUT_VARIABLE sloposk_qml_paths)
@@ -98,6 +99,7 @@ target_link_libraries(sloposk PRIVATE
     Qt6::DBus
     Qt6::WaylandClient
     LayerShellQt::Interface
+    KF6::WindowSystem
     PkgConfig::WAYLAND
 )