Implement issue #144 item 1: render markdown in user messages

[anaghnair]

Part of #144

Implements item 1 from the feature request: markdown rendering for user messages.

Summary

This updates user message rendering so markdown is displayed consistently on both sides of the chat.

• Wrap user message text in Streamdown
• Keep user message rendering static with isAnimating={false}
• Keep markdown controls hidden with controls={false}
• Leave the existing assistant streaming behavior intact

Verification

• npm run check
• Started the app locally with npm run dev
• Verified in Chrome using agent-browser skill
• Confirmed user-side markdown rendering for:
  • bold text
  • inline code
  • links
  • lists

Screenshots

Screenshots for the working fix are attached in the comments below.

[anaghnair]

Verification screenshots for issue #144 item 1:

Bold

Inline code

Link

List