Add iOS install hint + Apple PWA meta tags, fix missing beforeinstallprompt handler

iOS Safari has no beforeinstallprompt API, so the portal's Install
button (?install=1) silently did nothing there. IosInstallHint now
shows Share -> Add to Home Screen steps on Safari, or a prompt to
switch to Safari first if opened in another iOS browser/in-app
webview (those can't install PWAs on iOS at all).

Also added apple-mobile-web-app-title + apple-touch-icon so the
home screen icon isn't a page screenshot, and added the
beforeinstallprompt handler that was missing entirely (Android
Install button did nothing either).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -3,6 +3,15 @@ import ReactDOM from 'react-dom/client'
 import App from './App'
 import './index.css'
 
+// When opened from the portal's install button (?install=1), trigger the
+// PWA install prompt as soon as the browser offers it (Chrome/Edge only).
+if (new URLSearchParams(window.location.search).has('install')) {
+  window.addEventListener('beforeinstallprompt', e => {
+    e.preventDefault()
+    ;(e as Event & { prompt: () => Promise<void> }).prompt()
+  }, { once: true })
+}
+
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
     <App />