Adjust rest time from the watch with the same 1-second stepper

A gear on the watch root opens a small Settings sheet whose stepper
edits the shared restSeconds default (10-180s, 1s steps). Edits ride a
new settingsUpdate message to the phone - debounced per stepper burst,
falling back to transferUserInfo when unreachable - which clamps the
value, writes the shared default, and re-echoes it to every device
through the application context. While an edit is pending on the watch,
an in-flight context's stale rest value is skipped so it can't yank the
stepper back mid-edit.

CHANGELOG.md

@@ -4,6 +4,8 @@ Workouts is now on the Mac: a native app for managing routines and schedules and
 
 Rest time now adjusts in one-second steps — in Settings and in a routine's custom rest — instead of five-second jumps.
 
+Rest time can now be changed on the watch too — a new Settings sheet with the same one-second stepper, synced back to your iPhone.
+
 A live status panel between the timer and the figure now shows heart rate (with the target color cues, in much larger type), HR zone, calories, and workout time.
 
 A new Library tab puts your routines and the exercise library one tap away instead of buried in Settings.