Add missing type hints to public API classes in card_framework/v2

I noticed that several public-facing classes in `card_framework/v2` don’t currently have full type annotations on their methods.
Adding type hints would improve IDE autocomplete, static analysis, and overall developer experience, without changing any runtime behavior.

Suggested scope:

- `card_framework/v2/card.py`
- `card_framework/v2/header.py`
- `card_framework/v2/section.py`
- Common widgets like `text_paragraph.py`, `image.py`, etc.

This should be a purely additive change (type hints + imports from `typing`) and should not affect existing tests or serialization logic.

If this sounds reasonable, I’d be happy to take this up as a first contribution.