开放多项目大屏免登录访问

需求描述：/admin/display 需要无需管理员登录即可直接查看多项目公开运行状态。

实现思路：新增公开字段白名单接口与独立大屏页面，移除该路由的管理员会话依赖，同时保留 /api/admin/overview 的鉴权并补充隐私、路由和页面回归测试。

验证：Go 全量测试通过；前端 18 个测试文件共 59 项通过，TypeScript 检查通过。

web/src/pages/PublicDisplayCenterPage.tsx

@@ -0,0 +1,40 @@
+import { api } from "../api";
+import { FeedbackBanner, FreshnessBanner, LoadingState } from "../components/Feedback";
+import { usePollingResource } from "../hooks/usePollingResource";
+import { isTimestampStale } from "../lib/format";
+import { DisplayCenter } from "./AdminPage";
+
+export function PublicDisplayCenterPage() {
+  const resource = usePollingResource((signal) => api.displayOverview(signal), { intervalMs: 5_000 });
+  const data = resource.data;
+  const stale = Boolean(data) && isTimestampStale(resource.lastClientSuccessAt, 30_000);
+
+  return (
+    <div className="app-shell app-shell--admin app-shell--no-primary-action">
+      <a className="skip-link" href="#main-content">跳到主要内容</a>
+      <header className="app-header">
+        <div className="brand-lockup" aria-label="景区排队叫号系统">
+          <span className="brand-logo-frame" aria-hidden="true"><img className="brand-logo" src="/xiaoqikong-logo.jpg" alt="" /></span>
+          <span className="brand-lockup__text"><strong>景区排队叫号系统</strong></span>
+        </div>
+        <div className="account-menu"><span><strong>大屏中心</strong></span></div>
+      </header>
+      <main id="main-content" className="app-main">
+        <section className="admin-content" aria-label="公开大屏内容">
+          {resource.loading && !data ? <LoadingState label="正在汇总项目状态" /> : null}
+          {resource.error && !data ? (
+            <FeedbackBanner tone="danger" title="大屏数据暂时不可用" action={<button className="button button--secondary button--small" onClick={resource.refresh}>重试</button>}>
+              {resource.error.message}
+            </FeedbackBanner>
+          ) : null}
+          {data ? (
+            <>
+              <FreshnessBanner offline={resource.offline} stale={stale} timestamp={resource.lastClientSuccessAt} refreshing={resource.refreshing} errorMessage={resource.error?.message} onRetry={resource.refresh} />
+              <DisplayCenter projects={data.projects} />
+            </>
+          ) : null}
+        </section>
+      </main>
+    </div>
+  );
+}