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/tools/penpot_tree.py

@@ -6,7 +6,7 @@ a tree representation, which can be displayed or exported.
 """
 
 import re
-from typing import Any, Dict, Optional, Union, List
+from typing import Any, Dict, List, Optional, Union
 
 from anytree import Node, RenderTree
 from anytree.exporter import DotExporter