Add test_credentials.py for Penpot API credential verification and project listing

- Introduced a new script, `test_credentials.py`, to verify Penpot API credentials and list associated projects.
- The script loads environment variables, checks for required credentials, and attempts to authenticate with the Penpot API.
- Added functionality to fetch and display project details and files, including error handling for authentication and project retrieval.
- Updated `PenpotAPI` class to include a User-Agent header and improved error handling during profile retrieval.
- Minor adjustments in import order across various modules for consistency.

penpot_mcp/utils/http_server.py

@@ -2,9 +2,10 @@
 
 import io
 import json
+import socketserver
 import threading
 from http.server import BaseHTTPRequestHandler, HTTPServer
-import socketserver
+
 
 class InMemoryImageHandler(BaseHTTPRequestHandler):
     """HTTP request handler for serving images stored in memory."""