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/HealthKit/WorkoutLauncher.swift

@@ -0,0 +1,40 @@
+//
+// WorkoutLauncher.swift
+// Workouts
+//
+// Copyright 2025 Rouslan Zenetl. All Rights Reserved.
+//
+
+import Foundation
+import HealthKit
+
+/// Launches the companion Watch app into a workout when a session starts on the phone.
+///
+/// An iPhone app can't foreground its Watch app on its own — the only sanctioned path
+/// is HealthKit's `startWatchApp(toHandle:)`, which hands watchOS a workout
+/// configuration and brings the Watch app up to run a matching `HKWorkoutSession`.
+/// Calling it requires authorization to *share* workouts first.
+@MainActor
+final class WorkoutLauncher {
+    private let healthStore = HKHealthStore()
+
+    /// Strength/HIIT configuration handed to watchOS; the watch starts a session with
+    /// the same shape (see `WorkoutSessionManager`).
+    static func makeConfiguration() -> HKWorkoutConfiguration {
+        let configuration = HKWorkoutConfiguration()
+        configuration.activityType = .traditionalStrengthTraining
+        configuration.locationType = .indoor
+        return configuration
+    }
+
+    /// Ask watchOS to launch the Watch app into the workout. Best-effort: no-ops where
+    /// HealthKit is unavailable (e.g. iPad without it) and silently tolerates a missing
+    /// or unreachable paired watch.
+    func launchWatchWorkout() {
+        guard HKHealthStore.isHealthDataAvailable() else { return }
+        Task {
+            try? await healthStore.requestAuthorization(toShare: [.workoutType()], read: [])
+            try? await healthStore.startWatchApp(toHandle: Self.makeConfiguration())
+        }
+    }
+}