feat(dashboard): require basic auth for external clients only

- Allow direct access from localhost and private RFC1918 networks
- Enforce HTTP Basic Auth for non-private/external source IPs
- Read dashboard credentials from compose env vars
- Add .env.example entries for DASHBOARD_AUTH_USERNAME/PASSWORD
- Update README and DASHBOARD docs

docker-compose.yml

@@ -91,6 +91,8 @@ services:
       PALLADIUM_RPC_PORT: "2332"
       ELECTRUMX_RPC_HOST: "electrumx"
       ELECTRUMX_RPC_PORT: "8000"
+      DASHBOARD_AUTH_USERNAME: "${DASHBOARD_AUTH_USERNAME:-admin}"
+      DASHBOARD_AUTH_PASSWORD: "${DASHBOARD_AUTH_PASSWORD:-change-me-now}"
 
     volumes:
       - ./.palladium/palladium.conf:/palladium-config/palladium.conf:ro