Add PWA manifest support

Icons already existed in public/icons/ but nothing wired them up —
no manifest, no service worker, not installable.

frontend/package.json

@@ -25,6 +25,7 @@
     "@types/react-plotly.js": "^2.6.4",
     "@vitejs/plugin-react": "^4.2.1",
     "typescript": "^5.4.5",
-    "vite": "^5.2.11"
+    "vite": "^5.2.11",
+    "vite-plugin-pwa": "^1.3.0"
   }
 }

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: '/forecasting/',
-  plugins: [react()],
+  plugins: [
+    react(),
+    VitePWA({
+      registerType: 'autoUpdate',
+      manifest: {
+        name: 'Forecasting',
+        short_name: 'Forecasting',
+        start_url: '/forecasting/',
+        scope: '/forecasting/',
+        display: 'standalone',
+        theme_color: '#0077b6',
+        background_color: '#0077b6',
+        icons: [
+          { src: '/forecasting/icons/icon-192.png', sizes: '192x192', type: 'image/png' },
+          { src: '/forecasting/icons/icon-512.png', sizes: '512x512', type: 'image/png' },
+        ],
+      },
+      workbox: {
+        navigateFallback: '/forecasting/index.html',
+        navigateFallbackDenylist: [/\/api\//],
+        globPatterns: ['**/*.{js,css,html,ico,png,svg}'],
+      },
+    }),
+  ],
 })