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/Views/WorkoutLogs/WorkoutLogsView.swift

@@ -108,6 +108,7 @@ struct WorkoutLogsView: View {
 
 struct SplitPickerSheet: View {
     @Environment(SyncEngine.self) private var sync
+    @Environment(AppServices.self) private var services
     @Environment(\.dismiss) private var dismiss
 
     @Query(sort: [SortDescriptor(\Split.order), SortDescriptor(\Split.name)])
@@ -178,6 +179,8 @@ struct SplitPickerSheet: View {
         )
 
         Task { await sync.save(workout: doc) }
+        // Bring the Apple Watch up into the session so the user can run it from the wrist.
+        services.workoutLauncher.launchWatchWorkout()
         dismiss()
     }
 }