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/playlistmanager/PmFolderCard.qml

@@ -35,7 +35,7 @@ Rectangle {
     width: 160
     height: 150
     radius: 10
-    color: cardArea.containsMouse ? theme.surfaceHover : theme.surfaceElevated
+    color: cardArea.containsMouse ? theme.surfaceElevatedHover : theme.surfaceElevated
     opacity: (root.folder.isHidden === true) ? 0.6 : 1.0
 
     // padding 16 / spacing 8 / centred column.