Make cashup installable as a PWA

vite-plugin-pwa with manifest scoped to /cashup/, purple £ icons matching
portal style, auto install prompt when opened with ?install=1 from the
portal dashboard, nginx no-cache rules for sw.js and manifest.

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

frontend/src/main.tsx

@@ -3,6 +3,15 @@ import { createRoot } 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 })
+}
+
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
     <App />