feat: 后台增加开放消息开关

关闭后前台隐藏私聊/群聊入口并由后端拦截；同步去掉公告页重复计数胶囊。

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

frontend/lib/api.ts

@@ -596,6 +596,8 @@ export interface PublicSettings {
   allow_register: boolean;
   /** 全站是否开放评论；关闭后前台不展示评论能力 */
   allow_comments: boolean;
+  /** 全站是否开放私聊/群聊；关闭后前台不展示消息入口 */
+  allow_messages: boolean;
   post_cooldown_hours: number;
   /** 超过 N 行的代码块是否默认折叠；缺省 true */
   code_block_auto_fold: boolean;
@@ -620,6 +622,7 @@ const DEFAULT_PUBLIC_SETTINGS: PublicSettings = {
   site_description: "姜十三论坛 - 技术分享与讨论社区",
   allow_register: true,
   allow_comments: true,
+  allow_messages: true,
   post_cooldown_hours: 24,
   code_block_auto_fold: true,
   code_block_fold_lines: 5,
@@ -641,6 +644,7 @@ function normalizePublicSettings(data: Partial<PublicSettings> | null | undefine
         : DEFAULT_PUBLIC_SETTINGS.site_description,
     allow_register: data?.allow_register !== false,
     allow_comments: data?.allow_comments !== false,
+    allow_messages: data?.allow_messages !== false,
     post_cooldown_hours:
       typeof data?.post_cooldown_hours === "number" && data.post_cooldown_hours >= 0
         ? data.post_cooldown_hours
@@ -1744,6 +1748,7 @@ export type UpdateSiteSettingsBody = {
   site_description?: string;
   allow_register?: boolean;
   allow_comments?: boolean;
+  allow_messages?: boolean;
   post_cooldown_hours?: number;
   code_block_auto_fold?: boolean;
   code_block_fold_lines?: number;