fix(build): re-enable modulePreload polyfill for Safari < 17 compatibility

Disabling the polyfill broke chunk loading order on iOS 15 (Safari 15),
which does not support native modulepreload. The trip planner page —
the heaviest chunk graph in the app — went blank after the splash screen
because react-leaflet/react-leaflet-cluster chunks resolved out of order.
Other simpler pages were unaffected. The polyfill adds ~500 bytes and
self-disables in browsers with native support.

Fixes #1028

client/vite.config.js

@@ -90,7 +90,7 @@ export default defineConfig({
   ],
   build: {
     sourcemap: false,
-    modulePreload: { polyfill: false },
+    modulePreload: { polyfill: true },
   },
   server: {
     port: 5173,