fix: 私密广场首次加载默认 page=1，使搜索/标签过滤生效

私密广场标签页传 page: pages.private，初始为 undefined，被 JSON.stringify 丢弃后后端走非分页分支忽略 search/tag_ids。补上 ?? 1 兜底，与主标签页一致。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/components/workspace/agents/agent-gallery.tsx

@@ -112,7 +112,7 @@ export function AgentGallery() {
     search,
     tagIds,
     sort,
-    page: pages.private,
+    page: pages.private ?? 1,
     pageSize: PAGE_SIZE,
     enabled: activeTab === "private",
     owner: publisher?.id,