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.

Shared/Connectivity/WCPayload.swift

@@ -21,6 +21,7 @@ enum WCPayload {
 
     static let workoutUpdateType = "workoutUpdate"   // watch → phone (one workout)
     static let requestSyncType = "requestSync"       // watch → phone (please push state)
+    static let settingsUpdateType = "settingsUpdate" // watch → phone (a setting changed on the watch)
     static let liveProgressType = "liveProgress"     // watch → phone (ephemeral mirror frame)
     static let liveEndedType = "liveEnded"           // watch → phone (stop mirroring a run)
     static let liveHeartRateType = "liveHeartRate"   // watch → phone (ephemeral HR sample)
@@ -82,6 +83,16 @@ enum WCPayload {
 
     static func requestSyncMessage() -> [String: Any] { [typeKey: requestSyncType] }
 
+    // MARK: - Watch → Phone (a setting changed on the watch)
+
+    /// The watch's settings edits ride to the phone, which owns the durable value and
+    /// echoes it back to every device through the application context. Carries only the
+    /// settings the watch can edit (just rest time today); reuses the context's key, so
+    /// `decodeRestSeconds` reads both.
+    static func encodeSettingsUpdate(restSeconds: Int) -> [String: Any] {
+        [typeKey: settingsUpdateType, restSecondsKey: restSeconds]
+    }
+
     // MARK: - Watch → Phone (ephemeral live-run mirror)
 
     /// Anchor `Date`s ride as native plist values (not JSON), so they keep sub-second