Fix admin authentication by adding env vars to frontend container

The admin portal API routes run in the Next.js frontend container,
so ADMIN_USERNAME, ADMIN_PASSWORD, and JWT_SECRET need to be
available in the web service environment variables.

This fixes 401 Unauthorized errors when attempting to login.

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

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

docker-compose.yml

@@ -17,6 +17,10 @@ services:
       - VITE_API_BASE_URL=${VITE_API_BASE_URL}
       - VITE_WREN_API_TOKEN=${VITE_WREN_API_TOKEN}
       - VITE_STRIPE_PUBLISHABLE_KEY=${VITE_STRIPE_PUBLISHABLE_KEY}
+      # Admin Portal Authentication (server-side only)
+      - ADMIN_USERNAME=${ADMIN_USERNAME}
+      - ADMIN_PASSWORD=${ADMIN_PASSWORD}
+      - JWT_SECRET=${JWT_SECRET}
     restart: unless-stopped
     networks:
       - puffin-network