chore(mac): guard Darwin import for relay manager

apps/macos/Sources/Clawdis/RelayProcessManager.swift

@@ -1,6 +1,9 @@
 import Foundation
 import OSLog
 import Subprocess
+#if canImport(Darwin)
+import Darwin
+#endif
 
 @MainActor
 final class RelayProcessManager: ObservableObject {