Mirror a live Apple Watch run on a propped-up iPhone

Add an ephemeral live-run presence channel (separate from the durable
iCloud progress sync) so a propped-up iPhone can mirror the Watch's
Ready → work/rest → Finish flow in real time as the user swipes.

Watch drives, phone mirrors (read-only), so there's no echo loop:
- Watch's ExerciseProgressView broadcasts a LiveProgress frame on every
  phase transition (and an ended signal on leave) via sendMessage,
  reachable-only — throwaway presence, never written to iCloud.
- Timers ride as wall-clock anchors (Date kept native in the WC dict to
  preserve sub-second precision), so both devices count independently
  off shared start times and stay in lockstep without streaming ticks.
- Phone holds a transient LiveRunState; ContentView auto-presents a
  read-only LiveProgressMirrorView full-screen cover while a run is live.

Claude-Session: https://claude.ai/code/session_01SCv7zvGFcKy47KSTnTLxRe

CHANGELOG.md

@@ -1,5 +1,7 @@
 **June 2026**
 
+Prop your iPhone up during an Apple Watch workout and it now mirrors the live run — the same Ready → work/rest → Finish flow with running timers — following along set by set as you swipe on the watch.
+
 Editing an exercise or split on iPhone now steps the Apple Watch out of that workout, showing it as "Editing on iPhone" until you're done — so the watch never keeps running an exercise whose plan you're changing.
 
 The app now wears its signature purple: it's the accent color throughout, and a completed exercise is marked with a purple check. In-progress exercises now read as a neutral gray.