fix(nosignal): square the OSD (volume/brightness) to match the design

The volume/brightness OSD popped out as a rounded caelestia blob in the dynamic
surface colour. Square it + match the design glass:
- osd/Content: draw a flat square Theme.panelBg + 1px border background.
- ContentWindow: osdBg blob deformAmount 0 + radius 0 + opacity 0 (hidden;
  the OSD now owns its background).
- FilledSlider: track + handle radius -> 0 (Tokens.rounding.full ignored
  rounding.scale, so it stayed round).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

components/controls/FilledSlider.qml

@@ -16,7 +16,7 @@ Slider {
 
     background: StyledRect {
         color: Colours.layer(Colours.palette.m3surfaceContainer, 2)
-        radius: Tokens.rounding.full
+        radius: 0
 
         StyledRect {
             anchors.left: parent.left
@@ -51,7 +51,7 @@ Slider {
             anchors.fill: parent
 
             color: Colours.palette.m3inverseSurface
-            radius: Tokens.rounding.full
+            radius: 0
 
             MouseArea {
                 id: handleInteraction

modules/drawers/ContentWindow.qml

@@ -198,7 +198,9 @@ StyledWindow {
             id: osdBg
 
             panel: panels.osdWrapper
-            deformAmount: 0.25
+            deformAmount: 0 // NoSignal: square OSD (no organic blob deformation)
+            radius: 0
+            opacity: 0 // NoSignal: OSD draws its own flat square glass bg (osd/Content)
             x: panels.osdWrapper.x + panels.osd.x + root.borderThickness
             implicitWidth: panels.osd.width
         }

modules/osd/Content.qml

@@ -24,6 +24,15 @@ Item {
     implicitWidth: layout.implicitWidth + Tokens.padding.large + layout.anchors.horizontalCenterOffset * 2
     implicitHeight: layout.implicitHeight + Tokens.padding.large * 2
 
+    // NoSignal: flat square glass background matching the design popouts
+    Rectangle {
+        anchors.fill: parent
+        color: Theme.panelBg
+        radius: 0
+        border.width: 1
+        border.color: Theme.panelBorder
+    }
+
     ColumnLayout {
         id: layout