fix: Live Activity concurrency and state observation

- Add Sendable conformance to MusicActivityAttributes.ContentState
- Remove @preconcurrency on ActivityKit import
- Use nonisolated(unsafe) guards for Activity refs in task closures
- Add observeActivityState() to handle stale/ended/dismissed activity states
- Set staleDate (120s) instead of nil for push notification support

tabatago-swift/TabataGo/Models/MusicActivityAttributes.swift

@@ -1,7 +1,7 @@
 import ActivityKit
 
 struct MusicActivityAttributes: ActivityAttributes {
-    public struct ContentState: Codable, Hashable {
+    public struct ContentState: Codable, Hashable, Sendable {
         var title: String
         var artist: String
         var isPlaying: Bool