fix: Swift concurrency warning for ScrollOffsetPreferenceKey

Mark defaultValue as nonisolated(unsafe) to fix Xcode Cloud build error:
"Static property 'defaultValue' is not concurrency-safe because it is
nonisolated global shared mutable state"

ios_dashboard/OpenAgentDashboard/Views/Control/ControlView.swift

@@ -688,7 +688,7 @@ struct ControlView: View {
 // MARK: - Scroll Offset Preference Key
 
 private struct ScrollOffsetPreferenceKey: PreferenceKey {
-    static var defaultValue: CGFloat = 0
+    nonisolated(unsafe) static var defaultValue: CGFloat = 0
     static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
         value = nextValue()
     }