feat(slint): homologate menu surfaces with a defined muted border

Dropdown and context menus now sit on the content background
(surface-main) with a more defined border (border-muted, ~22% white)
instead of the elevated surface plus hairline border, so the popover
edge reads cleanly against the content instead of looking coarse.

cc

crates/qbz-slint/ui/primitives/ContextMenu.slint

@@ -6,10 +6,10 @@ import { Theme } from "../foundation/semantic-colors.slint";
 import { Radius } from "../foundation/radius.slint";
 
 export component ContextMenu inherits Rectangle {
-    background: Theme.surface-elevated;
+    background: Theme.surface-main;
     border-radius: Radius.sm;
     border-width: 1px;
-    border-color: Theme.border-subtle;
+    border-color: Theme.border-muted;
     drop-shadow-blur: 22px;
     drop-shadow-color: Theme.card-shadow;
     drop-shadow-offset-y: 6px;