[devtool] Add hydration diff indicator for diff lines (#88919)
Add a hydration diff indicator on the top right corner to tell users
which line is from client and which line is from server. Heard feedback
from users that sometimes they got confused about which line is from
where. Now they'll clear know that:
- `+ Client` shows `+` is client rendered content
- `- Server` shows `+` is server rendered content
<img width="579" height="225" alt="image"
src="https://github.com/user-attachments/assets/5285709a-044a-4d7f-87f9-08f1199eed4e"
/>
