fix: add cleanup to showTime timer effect to prevent stale timeout

Merged the separate cleanup useEffect into the showTime effect's
return function. This ensures the 5s timer is properly cleared
when remainingSeconds/totalSeconds change mid-countdown, preventing
a stale setShowTime(false) call after the component has moved to
a different phase.

Author: Aparencia

client/src/features/pomodoro/components/ImmersiveTimer.tsx

@@ -170,8 +170,13 @@ export default function ImmersiveTimer({
       if (showTimerRef.current) clearTimeout(showTimerRef.current);
       showTimerRef.current = setTimeout(() => setShowTime(false), 5000);
     }
+    return () => {
+      if (showTimerRef.current) {
+        clearTimeout(showTimerRef.current);
+        showTimerRef.current = null;
+      }
+    };
   }, [remainingSeconds, totalSeconds]);
-  useEffect(() => () => { if (showTimerRef.current) clearTimeout(showTimerRef.current); }, []);
 
   // 3.8 心流音乐引擎（工作阶段激活）
   const flowMusic = useFlowMusic(focusScore);