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

Workouts Watch App/Connectivity/WatchConnectivityBridge.swift

@@ -32,6 +32,7 @@ final class WatchConnectivityBridge: NSObject {
         // Apply whatever the phone last pushed, then ask for a fresh push.
         applyState(WCPayload.decodeSplits(session.receivedApplicationContext),
                    workouts: WCPayload.decodeWorkouts(session.receivedApplicationContext))
+        applyRestSeconds(session.receivedApplicationContext)
         requestSync()
     }
 
@@ -62,6 +63,12 @@ final class WatchConnectivityBridge: NSObject {
         }
     }
 
+    private func applyRestSeconds(_ dict: [String: Any]) {
+        if let rest = WCPayload.decodeRestSeconds(dict) {
+            UserDefaults.standard.set(rest, forKey: WCPayload.restSecondsKey)
+        }
+    }
+
     private func applyState(_ splits: [SplitDocument], workouts: [WorkoutDocument]) {
         guard !splits.isEmpty || !workouts.isEmpty else { return }
         var liveSplitIDs = Set<String>()
@@ -93,6 +100,10 @@ extension WatchConnectivityBridge: WCSessionDelegate {
     nonisolated func session(_ session: WCSession, didReceiveApplicationContext applicationContext: [String: Any]) {
         let splits = WCPayload.decodeSplits(applicationContext)
         let workouts = WCPayload.decodeWorkouts(applicationContext)
-        Task { @MainActor in self.applyState(splits, workouts: workouts) }
+        let rest = WCPayload.decodeRestSeconds(applicationContext)
+        Task { @MainActor in
+            self.applyState(splits, workouts: workouts)
+            if let rest { UserDefaults.standard.set(rest, forKey: WCPayload.restSecondsKey) }
+        }
     }
 }