fix: scroll stability, rendering, and tool-frame flicker

[DjagbleyEmmanuel]

Summary

Bundles the scroll/rendering/UX fixes from the fork's main branch into a single PR against upstream:

• Auto-scroll stickiness — chat now stays pinned to the bottom during streaming, while still allowing the user to manually scroll up without the viewport fighting them ( hook).
• Tool calling scroll — the tool-calling panel auto-scrolls to keep the latest step visible.
• Table/tool-frame border flicker — removes the visual flash of the border re-rendering on every streaming token.
• Reasoning scroll stability — stops the reasoning accordion from resetting its scroll position on each update.

Files changed

File	Change
web-app/src/hooks/useAutoScrollToBottom.ts	new — reusable scroll-to-bottom hook with manual-override detection
web-app/src/components/ai-elements/conversation.tsx	consume new hook
web-app/src/components/ai-elements/tools/tool.tsx	consume new hook for tool panel
web-app/src/containers/HtmlArtifact.tsx	consume new hook
web-app/src/containers/MessageItem.tsx	border flicker fix
web-app/src/styles/markdown.css	border flicker fix
web-app/src/routes/threads/$threadId.tsx	consume new hook