Fix national_rates.json deployment — copy to public/ directory

- Add missing data/national_rates.json fixture (was only in src/data/, not served by Vite)
- LoadingAnimation.jsx: Use import.meta.env.BASE_URL for national rates path
- Ensures file is available at /crdc-demo/data/national_rates.json on git-pages

src/components/LoadingAnimation.jsx

@@ -67,7 +67,7 @@ export default function LoadingAnimation({ district, state }) {
         )
 
         // ——— National rates (from static fixture, instant) ———
-        const nationalPromise = fetch('/data/national_rates.json')
+        const nationalPromise = fetch(import.meta.env.BASE_URL + 'data/national_rates.json')
           .then(r => r.json())
           .then(data => { /* cache for ChartPanel */ window.__NATIONAL_RATES__ = data })
           .catch(() => {})