fix(ui): give dropdown an opaque background

MenuBarExtra(.window) on the new SDK defaults to a vibrant translucent
material, letting whatever is behind bleed through. Apply
Color(.windowBackgroundColor) to the root VStack so the panel reads as
a normal opaque window in both light and dark mode.

release: 1.3.1 → 1.3.2

StockMonitor/Views/DropdownView.swift

@@ -103,5 +103,6 @@ struct DropdownView: View {
         }
         .padding(6)
         .frame(width: 320)
+        .background(Color(nsColor: .windowBackgroundColor))
     }
 }