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

README.md

@@ -15,8 +15,11 @@ your own iCloud Drive.
   timed exercises, and mark exercises complete.
 - **Progress tracking** — weight-progression charts per exercise across past
   sessions.
-- **Apple Watch companion** — start and run workouts from the wrist; changes sync
-  back to the phone.
+- **Apple Watch companion** — starting a workout on the iPhone launches the watch
+  app straight into it; run the session from your wrist as a HIIT cycle: count-up
+  work phases, count-down rests with final-three-second haptics and auto-advance,
+  and **One More** / **Done** on the last set. Rest time is configurable; changes
+  sync back to the phone.
 - **iCloud Drive sync** — your data lives as human-readable JSON in your iCloud
   Drive, synced across devices and visible in the Files app. iCloud is required.