This commit addresses two important issues:

1. Unicode Password Handling:
   - Added proper UTF-8 encoding in generate_key(), encrypt_file(), and decrypt_file() functions
   - Ensures unicode passwords (non-ASCII characters) work correctly
   - Added dedicated test for unicode password handling (test_unicode_password_internal)

2. Test Stability Improvements:
   - Updated test cases to use modern v4 config format
   - Refactored buffer overflow tests to focus on memory safety
   - Made large file tests more efficient with smaller test data
   - Used mocks for file permission tests to improve reliability

These changes improve password compatibility for international users and
ensure tests are more reliable in various environments.

🤖 Generated with [Claude Code](https://claude.ai/code

)

Co-Authored-By: Claude <noreply@anthropic.com>