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/Connectivity/PhoneConnectivityBridge.swift

@@ -42,9 +42,11 @@ final class PhoneConnectivityBridge: NSObject {
         wDesc.fetchLimit = 25
         let workouts = (try? context.fetch(wDesc)) ?? []
 
+        let restSeconds = UserDefaults.standard.object(forKey: WCPayload.restSecondsKey) as? Int ?? 45
         let payload = WCPayload.encodeState(
             splits: splits.map(SplitDocument.init(from:)),
-            workouts: workouts.map(WorkoutDocument.init(from:))
+            workouts: workouts.map(WorkoutDocument.init(from:)),
+            restSeconds: restSeconds
         )
         try? session.updateApplicationContext(payload)
     }