feat(ui): smooth streaming for chat messages

[karthikmudunuri]

Summary

Improve the streaming experience in the chat UI so that tokens appear with a smooth, natural cadence rather than arriving in visually jarring chunks.

Context

The current streaming implementation (landed in #123) renders tokens as they arrive from the backend, which can feel choppy — especially on slower connections or when the model emits tokens in bursts.

Desired behavior

• Tokens should animate into view with a smooth, typewriter-like cadence
• Buffer incoming tokens and flush them at a consistent visual rate
• Maintain low perceived latency (first token should still appear quickly)
• Smooth scrolling to keep the latest content in view during generation

Acceptance criteria

• Streaming text appears smoothly without visible chunking or jitter
• First-token latency is not noticeably degraded
• Auto-scroll remains smooth and doesn't jump
• Works correctly with markdown rendering and code blocks