feat(settings): Add usage badges to widget selector

Adds a visual indicator to the "Add Widget" dropdown in the Bar
settings panel to show which widgets are already in use and where.

- A small text badge ("L", "C", "R") now appears next to any widget
  that is already on a panel.
- The badges are reactive and update automatically when widgets are
  added or removed.
- This helps prevent accidental duplicate additions and makes widget
  management easier.

Modules/Settings/Extras/SectionEditor.qml

@@ -83,6 +83,7 @@ NBox {
       Item {
         Layout.fillWidth: true
       }
+
       NSearchableComboBox {
         id: comboBox
         model: availableWidgets