fix(qt): stop shadowing Item.enabled and Item.palette in four components

SettingsButton, QbzToggle and QbzSelect declared their own `enabled` over
Item's, and ReactiveRingsPanel its own `palette` over Item's QPalette: Qt
flagged all four at start (propertyCache: "overrides a member of the base
object"). The shadowed `enabled` never disabled input by itself — each
control filtered clicks and keys by hand, and any child that did not would
have kept receiving events. The inherited property now drives the visuals,
the focus policy and the guards, and disables the subtree the way Item
does; the rings' colour list is `ringPalette`.

cc

crates/qbz-qt/qml/controls/QbzSelect.qml

@@ -26,7 +26,6 @@ Rectangle {
     // window edge may opt into a lateral list without changing every other
     // QbzSelect instance. Supported values: "below" and "left".
     property string popupPlacement: "below"
-    property bool enabled: true
     property bool searchable: false
     // Bootstrap-style small variant (QbzSelect.slint:74-76). The default is the
     // standard Settings size; TOOLBARS opt in with `sm: true` — 30px tall, r6,