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/controls/BrowseGenreButton.qml

@@ -77,8 +77,8 @@ Rectangle {
     // .slint:77 — the same chip in three mounts). Accent and hover keep theirs.
     color: root.active ? theme.accent
          : genreArea.containsMouse
-             ? theme.surfaceHover
-             : (theme.ambientOn ? theme.surfaceElevatedA50 : theme.surfaceElevated)
+             ? theme.elevatedHoverFill
+             : theme.elevatedFill
 
     Row {
         id: genreRow