Fix: Pass FASTAPI_URL as build arg for Next.js rewrites

Next.js rewrites are evaluated at build time, not runtime.
Without FASTAPI_URL set during build, the rewrite destination
defaults to localhost:8000 which fails in Docker.

- Add FASTAPI_URL build arg to nextjs-app/Dockerfile
- Pass build arg in docker-compose.yml
- Pass build arg in Gitea Actions workflow

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitea/workflows/build-and-push.yml

@@ -89,6 +89,8 @@ jobs:
           push: ${{ gitea.event_name != 'pull_request' }}
           tags: ${{ steps.meta-frontend.outputs.tags }}
           labels: ${{ steps.meta-frontend.outputs.labels }}
+          build-args: |
+            FASTAPI_URL=http://backend:80/api
           # Cache disabled due to registry size limits
           # cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.FRONTEND_IMAGE_NAME }}:buildcache
           # cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.FRONTEND_IMAGE_NAME }}:buildcache,mode=max