Add per-split rest length and hands-free auto-advance flow

Two per-split settings, with the global Settings values as defaults:

- restSeconds: Int? — per-split rest, used between sets and (in flow) between
  exercises; nil falls back to the global default.
- autoAdvance: Bool? — flow mode: finishing an exercise rests, then opens the
  next one hands-free, all the way through the split.

Both are optional, snapshotted onto WorkoutDocument at the start sites (no live
split link), and not schema-bumped — same degradation pattern as activityType.

A thin RunFlowView wrapper (iOS + watch) owns the on-screen log and swaps it via
.id(currentLogID) on hand-off, so the per-exercise ExerciseProgressView stays
per-logID and untouched; the between-exercise rest reuses the existing .rest
countdown as the terminal page. The mirror reuses the per-logID live channel:
the wrapper suppresses the boundary .ended teardown so it follows across
exercises, and ContentView re-keys the cover on frame.logID — no sync-bridge
changes.

Morning Wake-Up ships as a flowing 45s-work / 15s-rest routine.

New Rest & Pacing section in the split editor exposes both controls.

Workouts/ContentView.swift

@@ -19,7 +19,11 @@ struct ContentView: View {
                 set: { presenting in if !presenting { liveRun.mute() } }
             )) {
                 if let frame = liveRun.presentable {
+                    // Re-key on the followed log so a flow-mode auto-advance (the driver
+                    // moving to the next exercise) rebuilds the mirror for the new exercise
+                    // instead of leaving a stale one pinned to the previous log.
                     LiveRunCoverView(frame: frame) { liveRun.mute() }
+                        .id(frame.logID)
                 }
             }
     }