add offline support

public/js/api-client.js

@@ -1,8 +1,33 @@
 (function (ns, $) {
   'use strict';
 
-  function request(path) {
-    return $.ajax({ url: ns.Config.apiBase + path, method: 'GET', dataType: 'json', cache: false });
+  async function offlineResponse(path) {
+    if (!ns.OfflineStore) return null;
+    if (path === '/health') {
+      const available = await ns.OfflineStore.state() || (await ns.OfflineStore.routes()).length;
+      return available ? { ok: true, offline: true } : null;
+    }
+    if (path.startsWith('/routes?')) {
+      if (!await ns.OfflineStore.state() && !(await ns.OfflineStore.routes()).length) return null;
+      return { routes: (await ns.OfflineStore.routes()).map(item => item.route), offline: true };
+    }
+    const response = await ns.OfflineStore.match(ns.Config.apiBase + path);
+    return response ? response.json() : null;
+  }
+
+  async function request(path) {
+    if (globalThis.navigator?.onLine === false) {
+      const response = await offlineResponse(path);
+      if (response) return response;
+    }
+    try {
+      return await $.ajax({ url: ns.Config.apiBase + path, method: 'GET', dataType: 'json', cache: false, timeout: 10000 });
+    } catch (error) {
+      if (error.status > 0) throw error;
+      const response = await offlineResponse(path);
+      if (response) return response;
+      throw error;
+    }
   }
 
   function routeQuery(position) {