feat: production-grade Live Activity with type-safe phases, decomposed views, previews, and alert transitions

- Replace raw string phase model with WorkoutPhase enum (Codable, Sendable, CaseIterable)
  with built-in .capitalized display name and SwiftUI .color per phase
- Decompose WorkoutLiveActivity into reusable view structs: PhasePill, CountdownText,
  WorkoutProgressBar, MusicInfoRow, HeartRateBadge, PhaseIndicatorDot, WorkoutLockScreenView,
  WorkoutSmallView — following CraftingSwift iOS 26 architecture patterns
- Add AlertConfiguration on work/rest/complete phase transitions so Dynamic Island
  expands and lights up at key moments
- Add 13 #Preview blocks across both widgets covering all presentation types:
  lock screen, expanded, compact, minimal — for instant Xcode Canvas feedback
- Add stale state handling (context.isStale shows 'Last updated' indicator)
- MusicLiveActivity: 5 new #Preview blocks for playing/paused/expanded/compact/minimal

tabatago-swift/TabataGo/Models/WorkoutActivityAttributes.swift

@@ -1,10 +1,44 @@
 import ActivityKit
 import Foundation
 
+#if canImport(SwiftUI)
+import SwiftUI
+#endif
+
+enum WorkoutPhase: String, Codable, Hashable, Sendable, CaseIterable {
+    case prep
+    case warmup
+    case work
+    case rest
+    case interBlockRest
+    case cooldown
+    case complete
+
+    var capitalized: String {
+        switch self {
+        case .interBlockRest: return "Inter-Block Rest"
+        default: return rawValue.capitalized
+        }
+    }
+
+    #if canImport(SwiftUI)
+    var color: Color {
+        switch self {
+        case .prep: return Color(red: 1.0, green: 0.58, blue: 0.0)
+        case .warmup: return Color(red: 1.0, green: 0.58, blue: 0.0)
+        case .work: return Color(red: 1.0, green: 0.42, blue: 0.21)
+        case .rest, .interBlockRest: return Color(red: 0.35, green: 0.78, blue: 0.98)
+        case .cooldown: return Color(red: 0.35, green: 0.78, blue: 0.98)
+        case .complete: return Color(red: 0.19, green: 0.82, blue: 0.35)
+        }
+    }
+    #endif
+}
+
 struct WorkoutActivityAttributes: ActivityAttributes {
     public struct ContentState: Codable, Hashable, Sendable {
         var exerciseName: String
-        var phase: String
+        var phase: WorkoutPhase
         var phaseEndDate: Date
         var roundCurrent: Int
         var roundTotal: Int