fix(qt): pane corners stay round under the dynamic background

AppShell hides its BezelCorner nubs while the field is meant to show
through the pane's corners, and Qt's clip is a rectangular scissor that
ignores radius — so any full-bleed child at y=0 squares the pane. The
doctrine was already written in AppShell.qml ("a full-bleed pane child
must round itself; there is no mask that can do it for it here"); these
eight surfaces predated it or, in the two thin bars added with the
ambient port, ignored it.

- The seven fixed 56px headers (Home, Search, DiscoverBrowse,
  ArtistReleases, PlaylistBrowse, PlaylistManager, PlayHistory) take
  topLeftRadius/topRightRadius. Invisible with the background off, where
  the view root paints the same colour beneath. PlaylistBrowse's SECOND
  bar is the category strip at mid-page and keeps square corners;
  PlayHistory keeps its unconditional opaque fill, which is reference
  parity (:43/:34) and a separate question from the corner.
- LocalAlbumView's neutral header band is now gated on the background,
  1:1 with the Qobuz page's atmosphere (AlbumView headerAtmoOn,
  AlbumPageView.slint:168): the two dark backdrops clash and the field
  already provides one. That band had been square since it was written;
  the nubs had been hiding it, and they only stand down when the field
  is showing. Header text there is theme.textPrimary, not the Qobuz
  page's light-on-dark hdrStrong, so it does not depend on the band to
  stay readable — checked before removing the backdrop from under it.

Gate: cargo build -p qbz-qt, both QML audits, offscreen smoke 0
complaints.

cc

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

@@ -407,6 +407,18 @@ Rectangle {
             // lost its backing exactly when it needed one.
             Rectangle {
                 anchors.fill: parent
+                // Rounded at the TOP because this bar is full-bleed at y=0 of
+                // the content pane, and under the dynamic background the pane's
+                // own rounding cannot reach it: Qt's `clip` is a rectangular
+                // scissor that ignores `radius`, and AppShell hides its bezel
+                // nubs while the field is meant to show through the corners.
+                // So a full-bleed pane child rounds ITSELF — AppShell.qml says
+                // exactly this ("there is no mask that can do it for it here")
+                // and this bar was the counterexample the owner spotted in
+                // Discover. Invisible with the background off: the view root
+                // paints the same colour underneath.
+                topLeftRadius: theme.radiusMd
+                topRightRadius: theme.radiusMd
                 color: root.ambientOn ? theme.surfaceMainA30 : theme.surfaceMain
             }