feat(web): refresh palette, simplify ranking toolbar and drop export

reaction_board/web/static/app.js

@@ -90,11 +90,13 @@ window.addEventListener("error", (event) => {
 async function boot() {
   renderNav();
   const banner = document.querySelector("#preview-banner");
+  const hint = document.querySelector("#topbar-hint");
   try {
     const payload = await health();
     const data = payload?.data || payload;
     if (data?.preview) {
       banner.hidden = false;
+      if (hint) hint.hidden = false;
     }
   } catch {
     toast("无法连接看板服务", "error");