feat(timestamp): 实现时间戳与日期时间转换功能

- 新增日期时间转时间戳组件 DatetimeToTimestamp
- 新增时间戳转日期时间组件 TimestampToDatetime
- 新增当前时间戳显示组件 TimestampExecution
- 重构 TimestampPage 页面，拆分功能到独立组件
- 扩展 timeUtils 工具函数，支持时区处理和格式化
- 添加时区选择器和时间单位切换功能
- 优化时间戳转换逻辑，支持毫秒和秒单位切换
- 改进 App.css 样式宽度设置

src/components/TimestampExecution.js

@@ -0,0 +1,45 @@
+import {useEffect, useState} from "react";
+import CopyButton from "./CopyButton";
+
+export function TimestampExecution() {
+  const [currentTimestamp, setCurrentTimestamp] = useState(Math.floor(Date.now()));
+  const [showMilliseconds, setShowMilliseconds] = useState(true);
+  const [isRunningTimestamp, setIsRunningTimestamp] = useState(true);
+  // 定时更新时间戳
+  useEffect(() => {
+    const timerId = isRunningTimestamp ? setInterval(() => {
+      setCurrentTimestamp(Math.floor(Date.now()));
+    }, showMilliseconds ? 100 : 1000) : null;
+    
+    return () => clearInterval(timerId);
+  }, [isRunningTimestamp, showMilliseconds]);
+  
+  const toggleUnit = () => setShowMilliseconds(prev => !prev);
+  const toggleTimestamp = () => setIsRunningTimestamp(prev => !prev);
+  
+  return (<div>
+    <h2>当前时间戳</h2>
+    <p>
+      <span>{Math.floor(currentTimestamp / (showMilliseconds ? 1 : 1000))}</span>
+      <span>{showMilliseconds ? '毫秒' : '秒'}</span>
+    </p>
+    <div>
+      <button type="button"
+              className="action-btn"
+              onClick={toggleUnit}
+              aria-label={showMilliseconds ? '切换为秒' : '切换为毫秒'}
+      >
+        切换单位
+      </button>
+      <CopyButton text={currentTimestamp.toString()} buttonText='复制'></CopyButton>
+      <button
+        type="button" // 明确指定类型，防止在 Form 中意外触发提交
+        className={`btn ${isRunningTimestamp ? 'stop-btn' : 'action-btn'}`}
+        onClick={toggleTimestamp}
+        aria-label={isRunningTimestamp ? '停止时间戳更新' : '开始时间戳更新'}
+      >
+        {isRunningTimestamp ? '停止' : '开始'}
+      </button>
+    </div>
+  </div>)
+}