Make rates installable as a PWA

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -9,6 +9,14 @@ const qc = new QueryClient({
   defaultOptions: { queries: { retry: 1, staleTime: 30_000 } },
 })
 
+
+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>
     <BrowserRouter basename="/rates">