fix: improve step management in TrainingCoachPage

- Refactored step clamping logic to ensure it correctly handles edge cases when the timeline is empty or when the unit changes.
- Updated useEffect to reset the step to 0 when the timeline is empty, enhancing user experience during training sessions.

frontend/src/pages/TrainingCoachPage.jsx

@@ -125,13 +125,23 @@ export default function TrainingCoachPage() {
 
   const timeline = useMemo(() => flattenPlanTimeline(unit), [unit])
 
+  const clampStep = (s, len = timeline.length) =>
+    Math.max(0, Math.min(s, Math.max(len - 1, 0)))
+
+  useEffect(() => {
+    if (!unit) return
+    if (timeline.length === 0) {
+      setStep(0)
+      return
+    }
+    setStep((prev) => clampStep(prev, timeline.length))
+  }, [unit, timeline.length])
+
+  const elapsedMs =
     pausedAccumMs + (runStartAt != null ? Date.now() - runStartAt : 0)
 
   const tickDisplaySec = Math.max(0, Math.floor(elapsedMs / 1000))
 
-  const clampStep = (s) =>
-    Math.max(0, Math.min(s, Math.max(timeline.length - 1, 0)))
-
   const currentEntry = timeline[step]
   const nextEntry = timeline[step + 1] || null
   const next2Entry = timeline[step + 2] || null