fix: serve cached server data before background sync

app/web/static/js/server-config.js

@@ -1,5 +1,6 @@
 const SERVER_STORAGE_KEY = "llm_monitor_servers";
 const ACTIVE_SERVER_KEY = "llm_monitor_active_server";
+const DATA_REFRESH_INTERVAL_MS = 30000;
 
 function normalizeHost(host) {
   if (!host) {
@@ -97,3 +98,56 @@ function buildServerUrl(path, serverId) {
   }
   return `${url.pathname}${url.search}`;
 }
+
+function getServerStorageKey(serverId, suffix) {
+  return `llm_monitor_${suffix}_${serverId}`;
+}
+
+function readServerCache(serverId, suffix) {
+  if (!serverId) {
+    return null;
+  }
+
+  const raw = localStorage.getItem(getServerStorageKey(serverId, suffix));
+  if (!raw) {
+    return null;
+  }
+
+  try {
+    return JSON.parse(raw);
+  } catch {
+    return null;
+  }
+}
+
+function writeServerCache(serverId, suffix, value) {
+  if (!serverId) {
+    return;
+  }
+
+  localStorage.setItem(getServerStorageKey(serverId, suffix), JSON.stringify(value));
+}
+
+function getCacheTimestamp(cacheValue) {
+  if (!cacheValue || !cacheValue.timestamp) {
+    return 0;
+  }
+
+  const parsed = Date.parse(cacheValue.timestamp);
+  return Number.isNaN(parsed) ? 0 : parsed;
+}
+
+function getLatestServerCacheTimestamp(serverId, suffixes) {
+  return suffixes.reduce((latest, suffix) => {
+    const value = readServerCache(serverId, suffix);
+    return Math.max(latest, getCacheTimestamp(value));
+  }, 0);
+}
+
+function isCacheStale(timestamp, maxAgeMs = DATA_REFRESH_INTERVAL_MS) {
+  if (!timestamp) {
+    return true;
+  }
+
+  return (Date.now() - timestamp) >= maxAgeMs;
+}