限制首页 Feed 自动加载上限，超出后支持手动加载更多历史帖子。

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

frontend/src/hooks/useForumLimits.ts

@@ -9,6 +9,9 @@ const DEFAULT_LIMITS: ForumLimitsPublic = {
   comment_max: 5000,
   search_keyword_min: 1,
   search_keyword_max: 50,
+  page_size_default: 30,
+  feed_max_pages: 10,
+  feed_max_items: 300,
   password_min_len: 6,
   avatar_max_mb: 2,
 };