refactor: extract basic_auth_header helper, add plaintext credential warnings

- basic_auth_header() in config.rs replaces 6 inline STANDARD.encode calls
- warn_plaintext_credentials() logs warning at startup if auth is in config.toml
- All protocol handlers use shared helper instead of duplicating base64 logic

nora-registry/src/main.rs

@@ -289,6 +289,9 @@ async fn run_server(config: Config, storage: Storage) {
     let storage_path = config.storage.path.clone();
     let rate_limit_enabled = config.rate_limit.enabled;
 
+    // Warn about plaintext credentials in config.toml
+    config.warn_plaintext_credentials();
+
     // Initialize Docker auth with proxy timeout
     let docker_auth = registry::DockerAuth::new(config.docker.proxy_timeout);