feat: 对话列表显示总数，SOUL 编辑区高度自适应收敛

- 新增 POST /threads/count：与 /threads/search 同一套过滤（标题搜索、
  系统会话排除），仅返回 {total}；ThreadMetaStore 增加 count 方法
  （SQL 走 COUNT(*)，JSON 过滤分批扫描计数）
- 对话页面搜索框下常驻显示「共 N 条对话」，分页条显示 当前页/总页数
  （useThreadsCount，仅随筛选条件变化，翻页不重复请求）
- 智能体右侧 SOUL 预览改为 40vh 上限自适应展开（不再全部展开）；
  详情面板/卡片编辑/创建页的 SOUL 输入框统一为适中高度 + 可拖拽

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend-web/src/core/threads/hooks.ts

@@ -841,6 +841,47 @@ export function useThreadsPage(options: {
   });
 }
 
+/**
+ * Total number of conversations matching the same filters as
+ * `useThreadsPage`, via the lightweight `/threads/count` endpoint — powers
+ * the "共 N 条 / X 页" display without fetching any rows. Depends only on the
+ * filters (not the page), so paging doesn't refire it.
+ */
+export function useThreadsCount(options: {
+  search?: string;
+  metadata?: Record<string, unknown>;
+}) {
+  const normalizedSearch = options.search?.trim() ?? "";
+  return useQuery<number>({
+    queryKey: [
+      "threads",
+      "search",
+      "count",
+      normalizedSearch,
+      options.metadata ?? null,
+    ],
+    queryFn: async () => {
+      const response = await apiFetch(
+        `${getLangGraphBaseURL()}/threads/count`,
+        {
+          method: "POST",
+          headers: { "Content-Type": "application/json" },
+          body: JSON.stringify({
+            metadata: options.metadata ?? {},
+            query: normalizedSearch || undefined,
+          }),
+        },
+      );
+      if (!response.ok) {
+        throw new Error("Failed to count conversations");
+      }
+      const data = (await response.json()) as { total: number };
+      return data.total;
+    },
+    refetchOnWindowFocus: false,
+  });
+}
+
 export function useDeleteThread() {
   const queryClient = useQueryClient();
   const apiClient = getAPIClient();