fix(proxy): make restart failures diagnosable and drop a fake server handle

- lib.rs: log an error instead of silently skipping the whole startup path
  when load_app_config() fails (previously a corrupt config left no trace).
- commands/proxy.rs: remove the unused `server_handle` field from
  ProxyServiceInstance that was populated with a dummy `tokio::spawn(async {})`.
  The real listener handle stays with AdminServerInstance.