feat(frontend): make API endpoint configurable via environment variable

Use nginx template support to allow API_URL to be passed at container
runtime, enabling the same image to be deployed to different environments.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/Dockerfile

@@ -21,8 +21,11 @@ FROM nginx:alpine
 # Copy built files
 COPY --from=build /app/dist /usr/share/nginx/html
 
-# Copy nginx config
-COPY nginx.conf /etc/nginx/conf.d/default.conf
+# Copy nginx template (processed at startup with envsubst)
+COPY nginx.conf.template /etc/nginx/templates/default.conf.template
+
+# Default API URL (override with -e API_URL=...)
+ENV API_URL=http://api:8000/
 
 EXPOSE 80