Fix CORS: add proxy support for browser-based API calls on git-pages

- Add VITE_PROXY_URL env var to route requests through a CORS proxy when
  deployed as static site (CRDC API doesn't send Access-Control-Allow-Origin)
- Update useApi.js, LoadingAnimation.jsx, and ChartPanel.jsx to respect proxy
- Add nginx.conf with /api/v1/ reverse proxy + CORS headers for Docker deployment
- Include proxy.php — simple PHP CORS proxy for git-pages hosts that support PHP
- Document CORS workaround in README

Without this fix, browser fetch() calls are blocked by same-origin policy and
the app shows a blank white screen despite serving correct HTML.

.gitea/workflows/pages.yml

@@ -32,6 +32,7 @@ jobs:
         run: |
           npm ci
           # VITE_API_BASE defaults to the production API; override if needed via secrets.
+          # For git-pages deployment, CORS is handled by setting up a proxy endpoint on the server.
           npm run build
 
       - name: Publish dist/ to pages branch