Make the open-panel mark find and fit its module

A center-anchored module is mounted twice: the copy that is drawn, and a
zero-size placeholder holding its place in the flow beside the anchor.
findPanelWidget returned whichever registered first, and that order is
not stable across a live bar reconfiguration, so a panel could open
anchored to the invisible copy -- mispositioned, with the drawn slot
never lighting up and switchPanelFrom unable to find it again.

The mark was also always 55% of the slot, which fits an icon but
underlines only a fraction of a text label, and runs the full height of a
multi-line module on a vertical bar. Modules can now say how long the
mark should be along the bar; anything that does not answer keeps the old
proportion.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

shell/Ui/WidgetButton.qml

@@ -59,6 +59,9 @@ Item {
   readonly property real scaledHorizontalMargin: Style.spaceReal(horizontalMargin)
   readonly property real scaledVerticalPadding: Style.spaceReal(verticalPadding)
   readonly property bool tooltipHovered: visible && interactive && !concealed && mouseArea.containsMouse
+  // Width of the painted label, for bar chrome that wants to line up with the
+  // text rather than with the slot it sits in. Zero on icon-only buttons.
+  readonly property real labelWidth: label.visible ? label.implicitWidth : 0
 
   visible: hasVisualContent || keepSpace
   opacity: !hasVisualContent || concealed ? 0 : (dimmed ? 0.45 : 1)