IEM geojson/sbw.geojson has CORS open to all origins so the browser can
fetch it directly. Moves all transformation logic into useStormEvents.js,
eliminating the need for vercel dev or any server-side proxy. Works with
plain pnpm run dev on localhost. Falls back to MOCK_STORM_EVENTS if IEM
is unreachable. api/storm-polygons.js kept as optional edge cache layer.