fix(progress): stamp the heartbeat with the application clock the page compares it with (#51)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/progress/tracker.go

@@ -37,7 +37,7 @@ type Reporter interface {
 // is for people, so a failed write is logged and never stops the job.
 type Sink interface {
 	SaveProgress(ctx context.Context, steps []domain.ProgressStep, at time.Time) error
-	Beat(ctx context.Context) error
+	Beat(ctx context.Context, at time.Time) error
 }
 
 const (
@@ -221,7 +221,7 @@ func (t *Tracker) Heartbeat(ctx context.Context, beatEvery, stillEvery time.Dura
 			return
 		case <-beat.C:
 			beatCtx, cancel := context.WithTimeout(ctx, t.saveTimeout)
-			err := t.sink.Beat(beatCtx)
+			err := t.sink.Beat(beatCtx, t.now())
 			cancel()
 			if err != nil && ctx.Err() == nil {
 				t.log.Warn("progress: heartbeat not saved", "err", err)