fix: Constrain images in markdown content with max-width: 100%

[Harshit28j]

fixes #165

Added max-width: 100% and height: auto CSS rules to images inside shiny-markdown-stream to ensure they scale responsively within their container.

Steps I took to reproduce this

from shiny.express import ui
from shinychat.express import Chat

ui.page_opts(title="Image Overflow Test")

chat = Chat(id="chat")

# Initial message with a wide image (this will overflow without max-width)
initial_message = """
Here are the latest blog posts:

## Latest Posts

1. **[Containerization in Posit Connect: Stable, Repeatable Deployments – No Dockerfile Required](https://posit.co/blog/containerization-in-posit-connect-stable-repeatable-deployments/)** (Dec 16)
   
   ![](https://posit.co/wp-content/uploads/2025/12/PST-Blog-Containerization-B.jpg)
   
   - By Kara Woo
   - Posit Connect will soon support containerization for building container images of deployed content
   - Enables stable, repeatable deployments without requiring Dockerfiles

2. **[Local models are not there (yet)](https://posit.co/blog/local-models-are-not-there-yet/)** (Dec 16)
   
   ![](https://posit.co/wp-content/uploads/2025/02/Blog-LocalModels-Dec2025-v2.jpg)
   
   - By Simon Couch & Sara Altman
   - Analysis showing that LLMs capable of running on laptops aren't yet powerful enough to effectively drive coding agents

3. **[Posit Workbench for Financial Services and Insurance](https://posit.co/blog/posit-workbench-for-financial-services-and-insurance/)** (Dec 15)
   
   ![](https://posit.co/wp-content/uploads/2025/12/Blog-WorkbenchFSI-Dec2025-v1.jpg)
   
   - By Posit Team
   - Highlighting how data science teams use Posit Workbench throughout the analytical lifecycle
   - Focus on financial services and insurance industries
"""

chat.ui(messages=[initial_message])

@chat.on_user_submit
async def handle_user_input(user_input: str):
    # Echo with an image to test
    response = f"""You said: {user_input}

Here's another image test:
![Wide image](https://via.placeholder.com/1200x400)
"""
    await chat.append_message(response)

result after fix (screenshot taken via dev tools)

[Harshit28j]

Hey @gadenbuie would appreciate your review on this whenever you get a chance🙂

[gadenbuie]

Thank you!