task: smoother streaming with media

[chloebyun-wd]

Add support for smoother media streaming:

• keeps chat pinned to latest content more reliably during stream
• handles post-render/media-size changes better for scrolling
• reduces jank during streaming updates
• avoids local storage pollution from incomplete/interrupted stream messages

[gemini-code-assist]

Code Review

This pull request refactors the chat scrolling and message streaming logic to improve the user experience and optimize message persistence. Key changes include the introduction of an isStreaming flag to defer message updates, a scrollToBottom helper, and an onMessageRendered callback to handle dynamic content like images. Review feedback highlights a bug where auto-scrolling is blocked during streaming, performance concerns regarding manual DOM manipulation and internal component definitions, and the need to refine message cleanup logic to prevent the loss of partial metadata (tools, artifacts) during stream interruptions.