feat(fasting): 新增轻断食功能模块

新增完整的轻断食功能，包括：
- 断食计划列表和详情页面，支持12-12、14-10、16-8、18-6四种计划
- 断食状态实时追踪和倒计时显示
- 自定义开始时间选择器
- 断食通知提醒功能
- Redux状态管理和数据持久化
- 新增tab导航入口和路由配置

hooks/useCountdown.ts

@@ -0,0 +1,54 @@
+import { useEffect, useMemo, useRef, useState } from 'react';
+import { formatCountdown } from '@/utils/fasting';
+
+interface CountdownOptions {
+  target: Date | null | undefined;
+  intervalMs?: number;
+  autoStart?: boolean;
+}
+
+export const useCountdown = ({ target, intervalMs = 1000, autoStart = true }: CountdownOptions) => {
+  const [now, setNow] = useState(() => new Date());
+  const timerRef = useRef<ReturnType<typeof setInterval> | null>(null);
+  const targetTimestamp = target instanceof Date ? target.getTime() : null;
+
+  useEffect(() => {
+    if (!autoStart) return undefined;
+    if (targetTimestamp == null) return undefined;
+
+    timerRef.current && clearInterval(timerRef.current);
+    timerRef.current = setInterval(() => {
+      setNow(new Date());
+    }, intervalMs);
+
+    return () => {
+      if (timerRef.current) {
+        clearInterval(timerRef.current);
+        timerRef.current = null;
+      }
+    };
+  }, [targetTimestamp, intervalMs, autoStart]);
+
+  useEffect(() => {
+    if (targetTimestamp == null) return;
+    setNow(new Date());
+  }, [targetTimestamp]);
+
+  const diffMs = useMemo(() => {
+    if (targetTimestamp == null) return 0;
+    return targetTimestamp - now.getTime();
+  }, [targetTimestamp, now]);
+
+  const formatted = useMemo(() => {
+    if (targetTimestamp == null) return '--:--:--';
+    return formatCountdown(new Date(targetTimestamp), now);
+  }, [targetTimestamp, now]);
+
+  return {
+    now,
+    diffMs,
+    formatted,
+    isExpired: diffMs <= 0,
+    hasTarget: targetTimestamp != null,
+  };
+};