fix(qt): elevated controls lift on hover instead of darkening

Buttons that rest on surfaceElevated used surfaceHover for their hover
fill: a 6% white wash meant for rows that rest transparent. Over an
elevated button it lands DARKER than the rest fill — on a modal's dark
ground the Cancel/Save/footer buttons went near-black under the pointer.

QbzTheme gains surfaceElevatedHover (a lift of surfaceElevated, derived
so every palette has it), its ambient half-alpha twin, and the
elevatedFill / elevatedHoverFill pair an ambient-aware control paints.
Every site with the old pairing (65 across modals, toolbars, selects,
circle actions, chips and cards) now hovers to the lift.

cc

crates/qbz-qt/qml/views/PlaylistView.qml

@@ -740,8 +740,8 @@ Rectangle {
                         // PlaylistView.slint:855-859 — resting fill
                         // translucent under the dynamic background.
                         color: sortArea.containsMouse
-                            ? theme.surfaceHover
-                            : (root.ambientOn ? theme.surfaceElevatedA50 : theme.surfaceElevated)
+                            ? theme.elevatedHoverFill
+                            : theme.elevatedFill
                         border.width: 1
                         border.color: theme.borderSubtle
                         Row {