fix(web): add @vitejs/plugin-react — dev mode needs the React Refresh runtime

TanStack Start's dev server requires a React Refresh plugin; without it `/@react-refresh`
404s, the client entry 500s, and nothing hydrates (blank screen — the production build was
unaffected since rollup handles JSX there). Pinned to the v4 line: plugin-react 6 imports
`vite/internal` (Vite 7 only) and we're on Vite 6. Must sit after tanstackStart() in the
plugin list.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/package.json

@@ -31,6 +31,7 @@
     "@types/node": "^22.10.0",
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
+    "@vitejs/plugin-react": "^4.7.0",
     "orval": "^8.16.0",
     "tailwindcss": "^4.0.0",
     "tw-animate-css": "^1.2.0",