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.

首先檢查 card_framework/v2/card.py、header.py、section.py 中的公開類別，以及 text_paragraph.py 和 image.py 等常用 widget。找出缺少註解的方法，並在進行增量修改前檢查附近的 typing 匯入和現有測試。完成的標準是：範圍內的公開 API 方法具有型別提示，且不改變序列化行為，同時現有測試仍然通過。

由索引模型根據 Issue 內容生成。