fix(qt): wheel-lock sweep across plain-Item modal overlays

Plain-Item overlays get no modal grab, so a MouseArea that swallows
presses still lets the wheel scroll the page underneath (the rule and
its wording live in DiscoverConfigModal.qml). Every scrim and panel
swallow across the modal set now accepts the wheel; the modal Popups
(TrackInfo, CastPicker, AlbumInfo, QconnectDev, LocalMediaInfo) already
had the Overlay grab, and CoverLightbox — a non-modal Popup — gets its
own.

cc

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

@@ -90,6 +90,8 @@ Item {
         MouseArea {
             anchors.fill: parent
             onClicked: QbzPlaylistImport.close()
+            // Wheel-lock (the DiscoverConfigModal rule).
+            onWheel: function (wheel) { wheel.accepted = true }
         }
     }
 
@@ -106,7 +108,11 @@ Item {
         border.width: 1
         border.color: theme.borderSubtle
         // Swallow clicks so they never reach the scrim.
-        MouseArea { anchors.fill: parent }
+        MouseArea {
+            anchors.fill: parent
+            // Wheel-lock (the DiscoverConfigModal rule).
+            onWheel: function (wheel) { wheel.accepted = true }
+        }
 
         readonly property int contentWidth: card.width - 48