Add PWA manifest support and icons

App had no manifest, no service worker, and no icons — not
installable as a PWA at all.

frontend/vite.config.ts

@@ -1,7 +1,31 @@
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
+import { VitePWA } from 'vite-plugin-pwa'
 
 export default defineConfig({
   base: '/twin-optimiser/',
-  plugins: [react()],
+  plugins: [
+    react(),
+    VitePWA({
+      registerType: 'autoUpdate',
+      manifest: {
+        name: 'Twin Optimiser',
+        short_name: 'Twin Optimiser',
+        start_url: '/twin-optimiser/',
+        scope: '/twin-optimiser/',
+        display: 'standalone',
+        theme_color: '#c9841a',
+        background_color: '#c9841a',
+        icons: [
+          { src: '/twin-optimiser/icons/icon-192.png', sizes: '192x192', type: 'image/png' },
+          { src: '/twin-optimiser/icons/icon-512.png', sizes: '512x512', type: 'image/png' },
+        ],
+      },
+      workbox: {
+        navigateFallback: '/twin-optimiser/index.html',
+        navigateFallbackDenylist: [/\/api\//],
+        globPatterns: ['**/*.{js,css,html,ico,png,svg}'],
+      },
+    }),
+  ],
 })