Add HIIT watch runner, rest-time setting, and HealthKit watch auto-launch

- Redesign the watch app into an active-workout runner: a root gate shows the
  in-progress workout's exercises or prompts to start one on iPhone, and each
  exercise runs as a horizontally-paged HIIT cycle (count-up work, count-down
  rest with final-three-second haptics + auto-advance, One More / Done on the
  last set). Replaces the old history list.
- Add a configurable rest-between-sets duration in iPhone Settings (default 45s),
  synced to the watch over WatchConnectivity.
- Launch the watch app into the session when a workout starts on the phone via
  HealthKit (startWatchApp); the watch runs an HKWorkoutSession for foreground
  runtime and ends it when the workout finishes. Adds the HealthKit entitlement +
  Health usage strings on both targets and WKBackgroundModes on the watch.

Claude-Session: https://claude.ai/code/session_018gg69MaUetDNzWzBXisfMV

Shared/Connectivity/WCPayload.swift

@@ -9,16 +9,18 @@ enum WCPayload {
     static let splitsKey = "splits"
     static let workoutsKey = "workouts"
     static let workoutKey = "workout"
+    static let restSecondsKey = "restSeconds"
 
     static let workoutUpdateType = "workoutUpdate"   // watch → phone (one workout)
     static let requestSyncType = "requestSync"       // watch → phone (please push state)
 
     // MARK: - Phone → Watch (application context: latest-state-wins)
 
-    static func encodeState(splits: [SplitDocument], workouts: [WorkoutDocument]) -> [String: Any] {
+    static func encodeState(splits: [SplitDocument], workouts: [WorkoutDocument], restSeconds: Int) -> [String: Any] {
         var dict: [String: Any] = [:]
         if let s = try? DocumentCoder.encoder.encode(splits) { dict[splitsKey] = s }
         if let w = try? DocumentCoder.encoder.encode(workouts) { dict[workoutsKey] = w }
+        dict[restSecondsKey] = restSeconds
         return dict
     }
 
@@ -32,6 +34,8 @@ enum WCPayload {
         return (try? DocumentCoder.decoder.decode([WorkoutDocument].self, from: data)) ?? []
     }
 
+    static func decodeRestSeconds(_ dict: [String: Any]) -> Int? { dict[restSecondsKey] as? Int }
+
     // MARK: - Watch → Phone (a single updated workout)
 
     static func encodeWorkoutUpdate(_ workout: WorkoutDocument) -> [String: Any] {