fix(qt): the sidebar playlists menu stops going translucent under ambient

Sidebar.slint:920-922 gives that panel the sidebar's OWN ambient treatment
(surface-card at 50%), and the port copied it. That made it the one menu
surface in the app that reads through the artwork while the dynamic background
is on. Every other menu and flyout paints flat and opaque: QbzContextMenu and
therefore CardMenu, SidebarRowMenu, ViewModeMenu and the audio flyout, plus
NavFlyout, SidebarFolderFlyout, QconnectFlyout, GenreFilterPopup and
LocalFilterPopup. Swept all of them; this was the only outlier.

A menu is text being read against art that moves underneath it, not chrome, so
the outlier loses.

cc

crates/qbz-qt/qml/shell/Sidebar.qml

@@ -715,9 +715,20 @@ Rectangle {
                     padding: 6
                     closePolicy: Popup.CloseOnPressOutside
                     background: Rectangle {
-                        // Matches this root's own ambient treatment
-                        // (Sidebar.slint:920-922).
-                        color: root.ambientOn ? theme.surfaceCardA50 : theme.surfaceCard
+                        // OPAQUE UNDER AMBIENT, unlike this root. Sidebar.slint
+                        // :920-922 gives the panel the sidebar's own ambient
+                        // treatment (surface-card @50%), and it was ported 1:1 —
+                        // which made this the ONE menu surface in the app that
+                        // goes translucent while the dynamic background is on.
+                        // Every other menu/flyout (QbzContextMenu and therefore
+                        // CardMenu / SidebarRowMenu / ViewModeMenu / the audio
+                        // flyout, plus NavFlyout, SidebarFolderFlyout,
+                        // QconnectFlyout, GenreFilterPopup, LocalFilterPopup)
+                        // paints a flat opaque surface. A menu is TEXT the user
+                        // is reading against artwork that moves under it, not
+                        // chrome, so the outlier lost: owner ruling 2026-08-19,
+                        // "la lectura se complica con la transparencia".
+                        color: theme.surfaceCard
                         radius: 8
                         border.width: 1
                         border.color: theme.borderSubtle