feat(hide): 密码可见隐藏块，发帖双栏预览与门禁体验修复

增加密码解锁与游客签名 cookie；发帖页对齐 1440 并默认双栏预览；修复 locked 解析、按钮对比度与回复聚焦。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/post/[id]/edit/page.tsx

@@ -37,7 +37,7 @@ export default async function EditPostPage({ params }: PageProps) {
   return (
     <div
       data-compose-viewport
-      className="j13-compose-page w-full max-w-6xl mx-auto"
+      className="j13-compose-page w-full"
     >
       <nav className="meta mb-4 flex items-center gap-1.5 flex-wrap text-[13px] shrink-0">
         <Link href="/" className="hover:text-[var(--accent)] transition-colors">