Add outline cursor mode to CursorSurface for slider rows

Wide content rows (slider rows in audio / monitor panels) opted out of
the cursor fill by setting fill: "transparent" on their CursorSurface,
which left them with no cursor visual at all — j/k landed there
silently with no way to know what was selected.

Add an `outline: bool` flag to CursorSurface. When true, hasCursor
paints an accent border (Style.focusBorderColor at
Style.focusBorderWidth) instead of a fill, leaving the row's chrome
fully visible underneath. Use for slider rows where a fill would
obscure the track.

Audio output / input slider rows and the monitor brightness row opt
in. The dev gallery's slider section replaces its bespoke Rectangle
wrapper with the same CursorSurface { outline: true }, so what the
gallery documents is what the panels ship.

shell/plugins/bar/widgets/monitorPanel.qml

@@ -415,7 +415,7 @@ Item {
               hasCursor: root.focusSection === "brightness" && root.selectedIndex === -1
               onHasCursorChanged: if (hasCursor) root.ensureCursorVisible(brightnessRow)
               foreground: root.bar.foreground
-              fill: "transparent"
+              outline: true
 
               Row {
                 id: brightnessInner