perf: replace always-on bargroup shadow with hover-only glow

the MultiEffect shadow on every BarGroup was causing 8+ offscreen FBO
renders on every bar redraw. now only activates on hover.

modules/BarGroup.qml

@@ -31,29 +31,10 @@ Item {
     implicitHeight: M.Theme.barHeight - 3 - _pad
 
     readonly property int _pad: 6
+    property bool _hovered: false
 
-    // Shadow source — rendered offscreen, only its glow is visible
-    Rectangle {
-        id: shadowSource
-        anchors.fill: parent
-        color: "transparent"
-        border.color: root.borderColor
-        border.width: 1
-        topLeftRadius: root._tlr
-        topRightRadius: root._trr
-        bottomLeftRadius: root._blr
-        bottomRightRadius: root._brr
-        visible: false
-    }
-
-    MultiEffect {
-        source: shadowSource
-        anchors.fill: shadowSource
-        shadowEnabled: true
-        shadowColor: root.borderColor
-        shadowBlur: 1.0
-        shadowVerticalOffset: 0
-        shadowHorizontalOffset: 0
+    HoverHandler {
+        onHoveredChanged: root._hovered = hovered
     }
 
     // Frosted base — semi-transparent so the bar background bleeds through
@@ -104,7 +85,7 @@ Item {
         }
     }
 
-    // Visible border
+    // Visible border — glow on hover only
     Rectangle {
         anchors.fill: parent
         color: "transparent"
@@ -114,6 +95,15 @@ Item {
         topRightRadius: root._trr
         bottomLeftRadius: root._blr
         bottomRightRadius: root._brr
+
+        layer.enabled: root._hovered
+        layer.effect: MultiEffect {
+            shadowEnabled: true
+            shadowColor: root.borderColor
+            shadowBlur: 1.0
+            shadowVerticalOffset: 0
+            shadowHorizontalOffset: 0
+        }
     }
 
     Row {