fix warnings

mac/Sources/Byre/Chat.swift

@@ -23,7 +23,7 @@ final class ChatModel {
     @ObservationIgnored private var offs: [() -> Void] = []
     @ObservationIgnored private var focusStart = true
     @ObservationIgnored private var visibleSummaries: Set<Int64> = []
-    @ObservationIgnored private var activeObserver: (any NSObjectProtocol)?
+    @ObservationIgnored private var activeObserver: NotificationCenter.ObservationToken?
     @ObservationIgnored private var started = false
 
     init(_ kind: ChatKind, _ store: Store) {
@@ -124,10 +124,8 @@ final class ChatModel {
         offs.append(feed.streamed.add { [weak self] ev in
             self?.builder.appendDelta(ev)
         })
-        activeObserver = NotificationCenter.default.addObserver(forName: NSApplication.didBecomeActiveNotification, object: nil, queue: .main) { [weak self] _ in
-            MainActor.assumeIsolated {
-                self?.markShown()
-            }
+        activeObserver = NotificationCenter.default.addObserver(of: NSApplication.self, for: .didBecomeActive) { [weak self] _ in
+            self?.markShown()
         }
     }