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/lib/api.ts

@@ -838,6 +838,15 @@ export async function apiUnlockPost(postId: string) {
   return res.json();
 }
 
+export async function apiUnlockPostPassword(postId: string, password: string) {
+  const res = await fetchWithRefresh(`/api/posts/${postId}/unlock-password`, {
+    method: "POST",
+    headers: clientHeaders({ "Content-Type": "application/json" }),
+    body: JSON.stringify({ password }),
+  });
+  return res.json();
+}
+
 export async function apiGetPoints(): Promise<{ points: number }> {
   const res = await fetchWithRefresh("/api/points", {
     method: "GET",