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>

backend/service/post_interact.go

@@ -1519,7 +1519,7 @@ func (s *PostService) loadManageablePost(actor *Actor, userID, postID uint) (*mo
 
 func (s *PostService) detailAfterInteract(post *model.Post, viewerID uint) (*PostDetail, error) {
 	detail := buildPostDetail(post)
-	sanitized, fullyLocked, hint := s.sanitizePostContent(post, viewerID, nil)
+	sanitized, fullyLocked, hint := s.sanitizePostContent(post, viewerID, nil, nil)
 	detail.Content = sanitized
 	detail.ContentLocked = fullyLocked
 	detail.AccessHint = hint