`cline-agent-chat-panel`: UI becomes unusable in long sessions due to unvirtualized message list

[nzneit]

What happened?

In sessions with many accumulated chat messages, the browser UI slows to a crawl and eventually becomes unresponsive. cline-agent-chat-panel.tsx renders every message as a live DOM node with no virtualization. Layout and paint cost scales linearly with total message history rather than the visible viewport, so long-running agent sessions progressively degrade until the app is unusable.

Steps to reproduce

1. Run one or more agent tasks through a long session (hundreds of messages accumulated)
2. Switch between task cards and observe chat panel lag
3. Scroll the message list while streaming is active

Scrolling, switching tasks, and general interaction become progressively sluggish. With enough history the app becomes effectively unusable.

Expected behavior

The UI remains responsive regardless of message history length.

Kanban version

0.1.68

Environment

All browsers and all OS

[linear-code]

CLINE-2295

[nzneit]

Proposed fix

Replace the messages.map(...) render with react-virtuoso's Virtuoso component, which is already a declared dependency in web-ui/package.json.

Key implementation notes:

• Virtuoso's followOutput prop replaces the current manual auto-scroll logic (isAutoScrollEnabled, useLayoutEffect, handleMessageListScroll, scrollContainerRef)
• atBottomThreshold maps to the existing BOTTOM_LOCK_THRESHOLD_PX = 24 constant
• <ClineThinkingIndicator /> and <ClineCreditLimitNotice /> (currently rendered inside the scroll container after the message list) must move to Virtuoso's components.Footer prop to preserve auto-scroll behavior
• computeItemKey={(index) => messages[index].id} for stable rendering
• The three auto-scroll behavior tests in cline-agent-chat-panel.test.tsx test internal scroll mechanics that Virtuoso replaces — these will need to be rewritten against Virtuoso's API or mocked at the library boundary

I'd like to take this on if the approach looks good.