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/Settings/SettingsView.swift

@@ -12,14 +12,27 @@ import IndieAbout
 struct SettingsView: View {
     @Environment(SyncEngine.self) private var sync
     @Environment(\.modelContext) private var modelContext
+    @Environment(AppServices.self) private var services
 
     @Query(sort: \Split.order) private var splits: [Split]
 
+    @AppStorage("restSeconds") private var restSeconds: Int = 45
     @State private var showingAddSplitSheet = false
 
     var body: some View {
         NavigationStack {
             Form {
+                // MARK: - Workout Section
+                Section(header: Text("Workout")) {
+                    Stepper(value: $restSeconds, in: 10...180, step: 5) {
+                        HStack {
+                            Text("Rest Between Sets")
+                            Spacer()
+                            Text("\(restSeconds)s").foregroundColor(.secondary)
+                        }
+                    }
+                }
+
                 // MARK: - Splits Section
                 Section(header: Text("Splits")) {
                     if splits.isEmpty {
@@ -94,6 +107,7 @@ struct SettingsView: View {
             .sheet(isPresented: $showingAddSplitSheet) {
                 SplitAddEditView(split: nil)
             }
+            .onChange(of: restSeconds) { _, _ in services.watchBridge.pushAll() }
         }
     }
 }