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/WorkoutSessionManager.swift

@@ -0,0 +1,78 @@
+//
+// WorkoutSessionManager.swift
+// Workouts Watch App
+//
+// Copyright 2025 Rouslan Zenetl. All Rights Reserved.
+//
+
+import Foundation
+import HealthKit
+import Observation
+
+/// Runs an `HKWorkoutSession` for the duration of a watch workout.
+///
+/// When the phone launches us via `startWatchApp(toHandle:)`, watchOS hands the app
+/// delegate a workout configuration; starting a session with it is what grants the
+/// freshly-launched app foreground runtime (and keeps it alive while the wrist is
+/// down). We don't record samples — the session exists purely to host the UI.
+@Observable
+@MainActor
+final class WorkoutSessionManager: NSObject {
+    private let healthStore = HKHealthStore()
+    private var session: HKWorkoutSession?
+
+    private(set) var isRunning = false
+
+    /// Prompt for workout-sharing authorization. Called once at launch so a later
+    /// phone-initiated launch can start a session without first hitting a permission
+    /// wall.
+    func requestAuthorization() {
+        guard HKHealthStore.isHealthDataAvailable() else { return }
+        healthStore.requestAuthorization(toShare: [.workoutType()], read: []) { _, _ in }
+    }
+
+    /// Start the session for a phone-launched workout. Idempotent — a second launch
+    /// while one is already running is ignored.
+    func start(with configuration: HKWorkoutConfiguration) {
+        guard HKHealthStore.isHealthDataAvailable(), session == nil else { return }
+        do {
+            let session = try HKWorkoutSession(healthStore: healthStore, configuration: configuration)
+            session.delegate = self
+            self.session = session
+            session.startActivity(with: Date())
+            isRunning = true
+        } catch {
+            session = nil
+            isRunning = false
+        }
+    }
+
+    /// End the session — called when the workout finishes (no active workout remains).
+    func end() {
+        session?.end()
+        clear()
+    }
+
+    private func clear() {
+        session = nil
+        isRunning = false
+    }
+}
+
+// MARK: - HKWorkoutSessionDelegate
+
+extension WorkoutSessionManager: HKWorkoutSessionDelegate {
+    nonisolated func workoutSession(
+        _ workoutSession: HKWorkoutSession,
+        didChangeTo toState: HKWorkoutSessionState,
+        from fromState: HKWorkoutSessionState,
+        date: Date
+    ) {
+        guard toState == .ended || toState == .stopped else { return }
+        Task { @MainActor in self.clear() }
+    }
+
+    nonisolated func workoutSession(_ workoutSession: HKWorkoutSession, didFailWithError error: Error) {
+        Task { @MainActor in self.clear() }
+    }
+}