[bug] WebChatPanel lags when typing with a long conversation history

Description

Typing in the WebChatPanel input becomes progressively slower as the conversation history grows. With a long chat, every keypress causes noticeable lag/jank.

Root cause

There are three compounding issues:

1. All markdown re-renders on every keystroke (primary cause)

input state lives in WebChatPanel alongside timeline.map(). Every keypress re-renders WebChatPanel, which walks the entire message list and re-runs react-markdown's full remark-gfm + rehype-raw AST pipeline for every assistant message. No message components are wrapped in React.memo.

Additionally, the components, remarkPlugins, and rehypePlugins props passed to <ReactMarkdown> are defined inline — new object/array references on every render — so ReactMarkdown cannot bail out even if the content string is unchanged.

2. No message list virtualization

@tanstack/react-virtual is already installed and used in AgentMemories.tsx, but never applied to any chat panel. All messages are always in the DOM.

3. DOM reflow per keystroke (textarea auto-resize)

FloatingChatInput has a useEffect([value]) dependency on the input string. On every character typed it removes the "input" event listener, calls adjustHeight() directly (forcing a height="auto" write → scrollHeight read → height write reflow cycle), then re-adds the listener.

Steps to reproduce

1. Start a long conversation with an agent (20+ message exchanges with markdown-heavy responses)
2. Click in the input field and type — lag increases proportionally to history length

Suggested fixes

• Wrap Markdown with React.memo and hoist remarkPlugins, rehypePlugins, and components to module-level constants so ReactMarkdown skips re-rendering when content is unchanged
• Move input state into FloatingChatInput so keystrokes only re-render the small input component, not WebChatPanel and its message list
• Change the useEffect([value]) to run once on mount ([]) and rely on a native "input" event listener for height adjustment — eliminating the reflow-per-keystroke
• (Follow-up) Apply useVirtualizer from @tanstack/react-virtual to the scroll container — the dependency is already installed

Start by locating WebChatPanel, Markdown, FloatingChatInput, and the AgentMemories.tsx use of @tanstack/react-virtual. Trace how input state and markdown props flow through the message list, then inspect the textarea height effect. Done means typing stays responsive with long markdown-heavy histories, while message rendering and input resizing avoid unnecessary per-keystroke work.