fix(pwa): detect upstream proxy auth challenges and recover gracefully

Behind Cloudflare Zero Trust or Pangolin, cross-origin auth redirects on
/api/* calls surface as CORS errors (error.response === undefined) that
the existing 401 interceptor never catches, leaving the PWA stuck with
network-error toasts instead of re-authenticating.

New connectivity module probes /api/health every 30s using fetch with
cache:no-store and inspects Content-Type to reliably detect whether the
server is reachable vs intercepted by an upstream proxy.

axios interceptor changes:
- On !error.response + navigator.onLine: run probeNow(); if the health
  probe also fails (proxy is intercepting all requests), trigger a guarded
  window.location.reload() so the edge proxy can intercept the top-level
  navigation and run its auth flow (covers CF Access and Pangolin 302 mode)
- On error.response status 401 with text/html body: same reload path,
  covering Pangolin header-auth extended compatibility mode which returns
  401+HTML instead of a 302 redirect. TREK own 401s are always JSON so
  there is no collision with the existing AUTH_REQUIRED branch.
- sessionStorage flag prevents reload loops; cleared on any successful
  response so the guard resets after re-auth.

/api/health excluded from SW NetworkFirst cache (vite.config.js regex)
and Cache-Control: no-store added server-side so probes always hit the
network and cannot be served stale from the 24h api-data cache.

LoginPage caches last-known appConfig in localStorage so the SSO button
renders in OIDC+UN/PW dual mode even when the config fetch is intercepted
by the proxy. Auto-redirect to IdP skipped when config comes from cache
to avoid redirect loops while the proxy is challenging.

Fixes discussion #836.

client/src/api/client.ts

@@ -1,5 +1,6 @@
 import axios, { AxiosInstance } from 'axios'
 import { getSocketId } from './websocket'
+import { isReachable, probeNow } from '../sync/connectivity'
 import en from '../i18n/translations/en'
 import br from '../i18n/translations/br'
 import de from '../i18n/translations/de'
@@ -69,10 +70,48 @@ export function isAuthPublicPath(pathname: string): boolean {
   return publicPaths.includes(pathname) || publicPrefixes.some((p) => pathname.startsWith(p))
 }
 
-// Response interceptor - handle 401, 403 MFA, 429 rate limit
+// Response interceptor - handle 401, 403 MFA, 429 rate limit, proxy auth challenges
 apiClient.interceptors.response.use(
-    (response) => response,
-    (error) => {
+    (response) => {
+      sessionStorage.removeItem('proxy_reauth_attempted')
+      return response
+    },
+    async (error) => {
+      // CF Access / Pangolin / similar: cross-origin redirect from /api/* surfaces
+      // as a CORS error with no response object. Probe the health endpoint to
+      // distinguish a proxy auth challenge from a genuine outage. If the server
+      // is reachable, a top-level reload lets the edge proxy run its auth flow.
+      if (!error.response && navigator.onLine) {
+        await probeNow()
+        // Both the original request and the health probe failed while the device
+        // has a network interface. This matches the proxy-auth-challenge pattern
+        // (CF Access / Pangolin intercept all requests and CORS-block XHR).
+        // A top-level reload lets the edge proxy intercept the navigation and
+        // run its auth flow. Guard with sessionStorage to prevent reload loops
+        // (server genuinely down would also land here, but only reloads once).
+        if (!isReachable()) {
+          const { pathname } = window.location
+          if (!isAuthPublicPath(pathname) && !sessionStorage.getItem('proxy_reauth_attempted')) {
+            sessionStorage.setItem('proxy_reauth_attempted', '1')
+            window.location.reload()
+            return Promise.reject(error)
+          }
+        }
+      }
+      // Pangolin header-auth extended compatibility mode: returns 401 with an
+      // HTML body (a JS redirect page) instead of a 302. TREK's own 401s are
+      // always application/json, so checking for text/html is unambiguous.
+      if (error.response?.status === 401) {
+        const ct = (error.response.headers?.['content-type'] as string | undefined) ?? ''
+        if (ct.includes('text/html')) {
+          const { pathname } = window.location
+          if (!isAuthPublicPath(pathname) && !sessionStorage.getItem('proxy_reauth_attempted')) {
+            sessionStorage.setItem('proxy_reauth_attempted', '1')
+            window.location.reload()
+            return Promise.reject(error)
+          }
+        }
+      }
       if (error.response?.status === 401 && (error.response?.data as { code?: string } | undefined)?.code === 'AUTH_REQUIRED') {
         const { pathname } = window.location
         if (!isAuthPublicPath(pathname)) {

client/src/main.tsx

@@ -3,6 +3,9 @@ import ReactDOM from 'react-dom/client'
 import { BrowserRouter } from 'react-router-dom'
 import App from './App'
 import './index.css'
+import { startConnectivityProbe } from './sync/connectivity'
+
+startConnectivityProbe()
 
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>

client/src/pages/LoginPage.tsx

@@ -117,15 +117,29 @@ export default function LoginPage(): React.ReactElement {
       return
     }
 
-    authApi.getAppConfig?.().catch(() => null).then((config: AppConfig | null) => {
-      if (config) {
-        setAppConfig(config)
-        if (!config.has_users) setMode('register')
-        if (!config.password_login && config.oidc_login && config.oidc_configured && config.has_users && !invite && !noRedirect) {
-          window.location.href = '/api/auth/oidc/login'
+    const CONFIG_CACHE_KEY = 'trek_app_config_cache'
+    authApi.getAppConfig?.()
+      .then((config: AppConfig) => {
+        try { localStorage.setItem(CONFIG_CACHE_KEY, JSON.stringify(config)) } catch { /* ignore quota errors */ }
+        return { config, fromCache: false }
+      })
+      .catch(() => {
+        try {
+          const raw = localStorage.getItem(CONFIG_CACHE_KEY)
+          return raw ? { config: JSON.parse(raw) as AppConfig, fromCache: true } : { config: null as AppConfig | null, fromCache: false }
+        } catch { return { config: null as AppConfig | null, fromCache: false } }
+      })
+      .then(({ config, fromCache }) => {
+        if (config) {
+          setAppConfig(config)
+          if (!config.has_users) setMode('register')
+          // Skip auto-redirect when config is from cache — network is unreliable
+          // and auto-redirecting to the IdP could loop if the proxy changed.
+          if (!fromCache && !config.password_login && config.oidc_login && config.oidc_configured && config.has_users && !invite && !noRedirect) {
+            window.location.href = '/api/auth/oidc/login'
+          }
         }
-      }
-    })
+      })
   }, [navigate, t, noRedirect])
 
   // Language detection chain (runs once on mount, only if user has no saved preference):

client/src/sync/connectivity.ts

@@ -0,0 +1,47 @@
+const PROBE_INTERVAL_MS = 30_000
+const PROBE_TIMEOUT_MS = 1_500
+
+let reachable = true
+const listeners = new Set<(v: boolean) => void>()
+
+function setReachable(v: boolean): void {
+  if (reachable === v) return
+  reachable = v
+  listeners.forEach(fn => fn(v))
+}
+
+async function probe(): Promise<void> {
+  if (!navigator.onLine) { setReachable(false); return }
+  try {
+    const ctrl = new AbortController()
+    const t = setTimeout(() => ctrl.abort(), PROBE_TIMEOUT_MS)
+    const res = await fetch('/api/health', {
+      method: 'GET',
+      credentials: 'include',
+      cache: 'no-store',
+      signal: ctrl.signal,
+    })
+    clearTimeout(t)
+    // /api/health returns JSON. CF Access / Pangolin will either return HTML
+    // (Pangolin 200 auth wall) or trigger a cross-origin redirect that throws
+    // below. Both proxy-auth scenarios resolve to reachable = false.
+    const ct = res.headers.get('content-type') || ''
+    setReachable(res.ok && ct.includes('application/json'))
+  } catch {
+    setReachable(false)
+  }
+}
+
+export function startConnectivityProbe(): void {
+  probe()
+  setInterval(probe, PROBE_INTERVAL_MS)
+  window.addEventListener('online', probe)
+  window.addEventListener('offline', () => setReachable(false))
+}
+
+export function isReachable(): boolean { return reachable }
+export function probeNow(): Promise<void> { return probe() }
+export function onChange(fn: (v: boolean) => void): () => void {
+  listeners.add(fn)
+  return () => listeners.delete(fn)
+}

client/vite.config.js

@@ -46,7 +46,7 @@ export default defineConfig({
           {
             // API calls — prefer network, fall back to cache
             // Exclude sensitive endpoints (auth, admin, backup, settings)
-            urlPattern: /\/api\/(?!auth|admin|backup|settings).*/i,
+            urlPattern: /\/api\/(?!auth|admin|backup|settings|health).*/i,
             handler: 'NetworkFirst',
             options: {
               cacheName: 'api-data',