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/SettingsPanel.qml

@@ -1,21 +1,25 @@
 import QtQuick
 import Sloposk
 
-// Replaces the keys while open. Binds straight to Settings, which persists.
-Rectangle {
+// Replaces the keys while open, over the window's own background so
+// opacity changes show live. Binds straight to Settings, which persists.
+Flickable {
     id: panel
 
     required property Settings settings
     required property LayoutModel layoutModel
 
-    color: Theme.background
-    radius: Theme.windowRadius
+    clip: true
+    contentHeight: columns.height + 2 * columns.y
+    boundsBehavior: Flickable.StopAtBounds
 
     Row {
         id: columns
         readonly property real columnWidth: (width - spacing) / 2
 
-        anchors { fill: parent; margins: 16 }
+        x: 16
+        y: 16
+        width: panel.width - 2 * x
         spacing: 32
 
         Column {
@@ -55,6 +59,24 @@ Rectangle {
                 checked: panel.settings.doubleSpacePeriod
                 onToggled: checked => panel.settings.doubleSpacePeriod = checked
             }
+
+            SectionHeader { text: qsTr("Appearance"); topPadding: 12 }
+
+            SliderRow {
+                width: parent.width
+                text: qsTr("Background")
+                from: 0.1 // Settings::MinBackgroundOpacity
+                value: panel.settings.backgroundOpacity
+                onMoved: value => panel.settings.backgroundOpacity = value
+            }
+
+            ToggleRow {
+                width: parent.width
+                text: qsTr("Blur Behind")
+                detail: qsTr("Frosted glass under a see-through background, if the desktop supports it")
+                checked: panel.settings.blurBackground
+                onToggled: checked => panel.settings.blurBackground = checked
+            }
         }
 
         Column {