Add type hints and docstrings to public methods

**Labels**: enhancement, good first issue 

**Body:** Add concise docstrings (Google or NumPy style) to `Library` and `Book` methods and ensure all public methods have type hints. 

**Acceptance Criteria:**

- Docstrings for `add_book`, `borrow_book`, `return_book`, `list_books`, `__init__`

- `mypy` passes with no errors (if configured later)