Relative URL handling w/ auto baseUrl inference

- Automatically infer baseUrl from object's @id when not provided
- Use document URL as fallback baseUrl when fetching remote objects
- Add comprehensive tests for relative URL resolution scenarios
- Update property fetching to use documentUrl for baseUrl inference
- Remove manual baseUrl parameter from property getter methods

This eliminates the need for manual baseUrl specification in most cases
while maintaining full compatibility with existing code.

https://github.com/fedify-dev/fedify/issues/411