feat(mail): 支持自定义邮件文件夹排序

- 后端为 `folders` 增加 `sort_order`，并提供文件夹重排接口。
- 前端邮件侧边栏支持自定义文件夹拖拽排序与乐观更新。
- 补充文件夹排序相关测试，并同步更新 API 类型定义。

apps/web/src/lib/api.ts

@@ -120,6 +120,10 @@ export const api = {
     const query = payload.mailboxId ? `?mailboxId=${encodeURIComponent(payload.mailboxId)}` : ""
     return request<MailFolder>(`/api/mail/folders${query}`, { method: "POST", body: JSON.stringify({ name: payload.name }) })
   },
+  reorderFolders: (payload: { mailboxId?: string; folderIds: string[]; folders?: { id: string; sortOrder: number }[] }) => {
+    const query = payload.mailboxId ? `?mailboxId=${encodeURIComponent(payload.mailboxId)}` : ""
+    return request<{ ok: boolean }>(`/api/mail/folders/reorder${query}`, { method: "POST", body: JSON.stringify(payload.folders ? { folders: payload.folders } : { folderIds: payload.folderIds }) })
+  },
   labels: (mailboxId?: string) => request<ListResponse<MailLabel>>(`/api/mail/labels${mailboxId ? `?mailboxId=${encodeURIComponent(mailboxId)}` : ""}`),
   createLabel: (payload: { mailboxId?: string; name: string; color?: string }) => {
     const query = payload.mailboxId ? `?mailboxId=${encodeURIComponent(payload.mailboxId)}` : ""