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/server/mcp_server.py

@@ -5,13 +5,14 @@ This module defines the MCP server with resources and tools for interacting with
 the Penpot design platform.
 """
 
+import argparse
 import hashlib
 import json
 import os
 import re
-import argparse
 import sys
-from typing import List, Optional, Dict
+from typing import Dict, List, Optional
+
 from mcp.server.fastmcp import FastMCP, Image
 
 from penpot_mcp.api.penpot_api import PenpotAPI