HeartbeatStore: fix main-actor cleanup

apps/macos/Sources/Clawdis/HeartbeatStore.swift

@@ -0,0 +1,29 @@
+import Foundation
+import SwiftUI
+
+@MainActor
+final class HeartbeatStore: ObservableObject {
+    static let shared = HeartbeatStore()
+
+    @Published private(set) var lastEvent: AgentRPC.HeartbeatEvent?
+
+    private var observer: NSObjectProtocol?
+
+    private init() {
+        self.observer = NotificationCenter.default.addObserver(
+            forName: AgentRPC.heartbeatNotification,
+            object: nil,
+            queue: .main
+        ) { [weak self] note in
+            guard let event = note.object as? AgentRPC.HeartbeatEvent else { return }
+            Task { @MainActor in
+                self?.lastEvent = event
+            }
+        }
+    }
+
+    @MainActor
+    deinit {
+        if let observer { NotificationCenter.default.removeObserver(observer) }
+    }
+}