fix(web): recover when the web app fails to start

docs/web.md

@@ -20,6 +20,8 @@ The web app is a progressive web app, so Chrome and Edge offer to install it to
 - The service worker fetches pages from the network first and falls back to the cache when offline, so a new deployment is never stuck behind an old copy. Content-hashed bundles and assets under `/_expo/static/` and `/assets/` are cached once and reused. Bluetooth still needs the PC nearby; offline only means the app opens.
 - `vercel.json` serves `sw.js` and the manifest with `no-cache`, and the hashed bundles as immutable. To change caching behaviour, change the `CACHE` name in `sw.js` so old caches are cleared on activation.
 
+If the app has not started 10 seconds after the page loads, an inline script in `src/app/+html.tsx` shows "Switchify Remote didn't start" with a Reload button. Reload unregisters the service worker and clears its caches first, so a broken or stale copy cannot leave anyone on the loading screen. The root layout reports that the app started, which removes the message if a slow connection made it appear. It only runs in production exports.
+
 ## Switch scanning
 
 Switch users can operate Remote with keyboard-style switch interfaces, which send key presses such as Space or Enter. Scanning uses [@switchify/scanning](https://github.com/switchifyapp/switchify-scanning), the TypeScript port of Switchify PC's item scanner, so timing, passes, groups and hold actions behave as they do on the PC.

src/app/+html.tsx

@@ -11,6 +11,48 @@ if ('serviceWorker' in navigator && window.isSecureContext) {
   });
 }`;
 
+/**
+ * If the app has not started 10 seconds after the page loads, offer a reload that
+ * also drops the service worker and its caches, so a broken or stale copy can never
+ * leave someone on the static loading screen. The app removes it once it starts.
+ */
+const startupWatchdog = `
+(function () {
+  var started = false;
+  window.__switchifyStarted = function () {
+    started = true;
+    var shown = document.getElementById('switchify-start-failed');
+    if (shown) shown.remove();
+  };
+  function reload() {
+    var work = [];
+    if (navigator.serviceWorker) work.push(navigator.serviceWorker.getRegistrations().then(function (all) { return Promise.all(all.map(function (r) { return r.unregister(); })); }));
+    if (window.caches) work.push(caches.keys().then(function (keys) { return Promise.all(keys.map(function (k) { return caches.delete(k); })); }));
+    Promise.all(work).catch(function () {}).then(function () { location.reload(); });
+  }
+  setTimeout(function () {
+    if (started || !document.body) return;
+    var box = document.createElement('div');
+    box.id = 'switchify-start-failed';
+    box.setAttribute('role', 'alert');
+    box.style.cssText = 'position:fixed;inset:0;z-index:2147483647;display:flex;flex-direction:column;align-items:center;justify-content:center;gap:16px;padding:24px;background:#0B0B0D;color:#F7F7F8;font:16px/1.5 system-ui,sans-serif;text-align:center';
+    var title = document.createElement('p');
+    title.style.cssText = 'margin:0;font-size:22px;font-weight:600';
+    title.textContent = 'Switchify Remote didn’t start';
+    var body = document.createElement('p');
+    body.style.cssText = 'margin:0;max-width:28em;color:#B4B4BE';
+    body.textContent = 'Reload to fetch a fresh copy. Your saved PCs and settings are kept.';
+    var button = document.createElement('button');
+    button.type = 'button';
+    button.textContent = 'Reload';
+    button.style.cssText = 'font:inherit;font-weight:600;min-height:52px;padding:0 32px;border:0;border-radius:12px;background:#D90429;color:#fff';
+    button.addEventListener('click', reload);
+    box.append(title, body, button);
+    document.body.appendChild(box);
+    button.focus();
+  }, 10000);
+})();`;
+
 export default function Root({ children }: PropsWithChildren) {
   return (
     <html lang="en">
@@ -23,6 +65,7 @@ export default function Root({ children }: PropsWithChildren) {
         <link rel="manifest" href="/manifest.json" />
         <link rel="apple-touch-icon" href="/icons/apple-touch-icon.png" />
         <ScrollViewStyleReset />
+        {process.env.NODE_ENV === 'production' ? <script dangerouslySetInnerHTML={{ __html: startupWatchdog }} /> : null}
         {process.env.NODE_ENV === 'production' ? <script dangerouslySetInnerHTML={{ __html: registerServiceWorker }} /> : null}
       </head>
       <body>{children}</body>

src/app/_layout.tsx

@@ -1,3 +1,4 @@
+import { useEffect } from 'react';
 import { DarkTheme, DefaultTheme, Stack, ThemeProvider as NavigationThemeProvider } from 'expo-router';
 import { StatusBar } from 'expo-status-bar';
 import { SafeAreaProvider } from 'react-native-safe-area-context';
@@ -7,13 +8,14 @@ import { ConnectionProvider } from '@/connection/ConnectionContext';
 import { diagnosticsScreenOptions } from '@/navigation/diagnosticsScreenOptions';
 import { FirstRunSetupGate } from '@/onboarding/FirstRunSetup';
 import { StoreCaptureRoot, storeCaptureEnabled } from '@/store-capture/StoreCaptureRoot';
-import { installPlatform } from '@/platform/installPlatform';
+import { installPlatform, markAppStarted } from '@/platform/installPlatform';
 import { ScanningProvider } from '@/scanning/ScanningProvider';
 import { ThemeProvider, useTheme } from '@/theme/ThemeContext';
 
 installPlatform();
 
 export default function RootLayout() {
+  useEffect(() => { markAppStarted(); }, []);
   return (
     <SafeAreaProvider>
       <ThemeProvider>{storeCaptureEnabled() ? <StoreCaptureRoot /> : <ScanningProvider><ThemedApp /></ScanningProvider>}</ThemeProvider>

src/platform/installPlatform.ts

@@ -1 +1,3 @@
 export function installPlatform(): void {}
+
+export function markAppStarted(): void {}

src/platform/installPlatform.web.test.ts

@@ -1,7 +1,7 @@
 /** @jest-environment jsdom */
 import { AccessibilityInfo, Alert } from 'react-native';
 
-import { installPlatform } from './installPlatform.web';
+import { installPlatform, markAppStarted } from './installPlatform.web';
 
 describe('web platform shims', () => {
   beforeAll(() => installPlatform());
@@ -46,4 +46,13 @@ describe('web platform shims', () => {
     expect(keep).toHaveBeenCalledTimes(1);
     expect(unpair).toHaveBeenCalledTimes(1);
   });
+
+  it('tells the startup watchdog the app has started', () => {
+    const started = jest.fn();
+    (window as Window & { __switchifyStarted?: () => void }).__switchifyStarted = started;
+    markAppStarted();
+    expect(started).toHaveBeenCalledTimes(1);
+    delete (window as Window & { __switchifyStarted?: () => void }).__switchifyStarted;
+    expect(() => markAppStarted()).not.toThrow();
+  });
 });

src/platform/installPlatform.web.ts

@@ -90,3 +90,9 @@ export function installPlatform(): void {
     else document.addEventListener('DOMContentLoaded', () => liveRegion(), { once: true });
   }
 }
+
+/** Tells the root HTML's startup watchdog that the app rendered, so it never shows its recovery message. */
+export function markAppStarted(): void {
+  if (typeof window === 'undefined') return;
+  (window as Window & { __switchifyStarted?: () => void }).__switchifyStarted?.();
+}