fix(header): 用 new Date().getTime() 避免 react-hooks/purity 报错

Date.now() 被 react-hooks/purity 规则判为 render 期不纯调用，
改用与原实现一致的 new Date().getTime()。

Author: claude

app/components/Header.tsx

@@ -12,7 +12,7 @@ import { LiveDate } from "./LiveDate";
 
 export async function Header() {
   const t = await getTranslations("header");
-  const editionTimestampMs = Date.now();
+  const editionTimestampMs = new Date().getTime();
   return (
     <header className="fixed top-0 w-full z-50 bg-[var(--background)] border-b border-[var(--foreground)] py-2 transition-colors duration-300">
       <div className="container mx-auto px-6">