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.

tests/test_penpot_tree.py

@@ -7,14 +7,14 @@ import pytest
 from anytree import Node, RenderTree
 
 from penpot_mcp.tools.penpot_tree import (
-    build_tree, 
-    print_tree, 
-    export_tree_to_dot, 
-    find_page_containing_object, 
-    find_object_in_tree, 
-    convert_node_to_dict, 
+    build_tree,
+    convert_node_to_dict,
+    export_tree_to_dot,
+    find_object_in_tree,
+    find_page_containing_object,
     get_object_subtree,
-    get_object_subtree_with_fields
+    get_object_subtree_with_fields,
+    print_tree,
 )