Add Warm-Up and Stretching activity types

Adds WorkoutActivityType.warmUp (HealthKit .preparationAndRecovery) and
.stretching (.flexibility), and retags the six starter splits that were all
mislabeled as Functional Strength:

- Warm-Up:    Upper Body Warm-Up, Lower Body Warm-Up, Morning Wake-Up
- Stretching: Morning Mobility, Full Body Stretch, Evening Stretch

The split editor's activity picker surfaces them automatically (CaseIterable).
Older app versions decode the new raw values as the default type — additive and
not schema-gated, so no quarantine.

Shared/Model/Enums.swift

@@ -47,6 +47,8 @@ enum WorkoutActivityType: Int, CaseIterable, Codable, Sendable {
     case coreTraining = 3
     case cardio = 4
     case cycling = 5
+    case warmUp = 6
+    case stretching = 7
 
     var displayName: String {
         switch self {
@@ -56,6 +58,8 @@ enum WorkoutActivityType: Int, CaseIterable, Codable, Sendable {
         case .coreTraining: "Core Training"
         case .cardio: "Cardio"
         case .cycling: "Cycling"
+        case .warmUp: "Warm-Up"
+        case .stretching: "Stretching"
         }
     }
 
@@ -67,6 +71,8 @@ enum WorkoutActivityType: Int, CaseIterable, Codable, Sendable {
         case .coreTraining: "figure.core.training"
         case .cardio: "figure.run"
         case .cycling: "figure.outdoor.cycle"
+        case .warmUp: "figure.arms.open"
+        case .stretching: "figure.flexibility"
         }
     }
 }