fix: move Live Activity ownership to ViewModel, fix timer-at-0 and background freeze

**Architecture (PlayerViewModel):**
- Move ActivityKit lifecycle from SwiftUI View to ViewModel (MVVM correction)
- call syncActivity() at END of enterPhase() — after all state is set,
  eliminating the race where phase was Published before timeRemaining
- Always recalculate phaseEndDate = Date() + timeRemaining (no stale cache)
- Dedicated Timer in ViewModel for periodic heart-rate/track sync (5s)
- Start/stop activity sync timer on play/pause/resume/abandon/finish
- stale activity reference discard + recreate-on-failure fallback
- Modern iOS 16.2+ API: ActivityContent, non-throwing update()

**PlayerView:**
- Remove all ActivityKit code (import, @State workoutActivity,
  phaseEndDate, dynamicIslandAvailable, 4 methods, .onReceive timer)
- Delegate to ViewModel: onChange(musicVM.currentTrack) sets vm.trackTitle/Artist
  and calls vm.syncActivity(); onDisappear calls await vm.endActivity()
- Music/audio onChange handlers no longer contain activity logic

**Info.plist:**
- Add UIBackgroundModes → audio so music continues and app stays alive
  in background, allowing Timer-based activity updates
- Widget Info.plist: add NSSupportsLiveActivitiesFrequentUpdates

**WorkoutActivityAttributes.ContentState:**
- Add Sendable conformance for Swift 6 strict concurrency

Fixes: timer stuck at 0 on first work phase, exercise name missing,
       music stopping in background, Dynamic Island freezing in background,
       widget drift due to cached phaseEndDate

tabatago-swift/TabataGo/Models/WorkoutActivityAttributes.swift

@@ -2,7 +2,7 @@ import ActivityKit
 import Foundation
 
 struct WorkoutActivityAttributes: ActivityAttributes {
-    public struct ContentState: Codable, Hashable {
+    public struct ContentState: Codable, Hashable, Sendable {
         var exerciseName: String
         var phase: String
         var phaseEndDate: Date