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>

qml/Main.qml

@@ -18,6 +18,10 @@ Window {
     readonly property rect inputRegion: keyboard.alternates.open
         ? Qt.rect(0, 0, width, height)
         : Qt.rect(panel.x, panel.y, panel.width, panel.height)
+    // Empty when nothing shows through, so the compositor skips the blur.
+    readonly property rect blurRegion: settings.blurBackground && settings.backgroundOpacity < 1
+        ? Qt.rect(panel.x, panel.y, panel.width, panel.height)
+        : Qt.rect(0, 0, 0, 0)
 
     width: 1000
     height: 340 + popupMargin
@@ -27,13 +31,17 @@ Window {
     color: "transparent"
 
     onInputRegionChanged: placement?.setInputRegion(inputRegion)
-    onPlacementChanged: placement?.setInputRegion(inputRegion)
+    onBlurRegionChanged: placement?.setBlurRegion(blurRegion, Theme.windowRadius)
+    onPlacementChanged: {
+        placement?.setInputRegion(inputRegion);
+        placement?.setBlurRegion(blurRegion, Theme.windowRadius);
+    }
 
     Rectangle {
         id: panel
         anchors { fill: parent; topMargin: root.popupMargin }
         radius: Theme.windowRadius
-        color: Theme.background
+        color: Qt.rgba(Theme.background.r, Theme.background.g, Theme.background.b, root.settings.backgroundOpacity)
 
         Keyboard {
             id: keyboard