Chat: memoise item views so a streamed text delta re-renders only the last item #924

Goal

A long chat stays responsive while an answer streams: each text delta re-renders only the item it changes, not every earlier item with its Markdown.

Context

On develop the frontend uses no React.memo at all. ChatPanel.tsx (:676–684) copies the whole items array on every chat-text-delta, and items.map (:1476) renders every ChatItemView, including react-markdown for each assistant message. After the chat panel series (#832–#847) the item views live in src/components/chat/ and the transcript reducer keeps the object identity of unchanged items.

Memoising was not safe before: DocumentCard reads the document-action registry with listDocumentActions() during render, and extension contributions registered later only appeared because the whole chat re-rendered. The reactive registry (#884) removes that dependency.

Requirements

• ChatItemView (and AssistantMessageView/DocumentCard if they are rendered separately) is wrapped in React.memo; the props passed from the panel are stable for unchanged items (callbacks via useCallback or a stable dispatch, no inline objects).
• DocumentCard reads document actions through the reactive registry (useSyncExternalStore subscription), so a document action registered or removed after the card was rendered still appears or disappears.
• No visible behaviour change.

Acceptance Criteria

• Test with the React Profiler (or a render counter in a test-only wrapper): a chat with 200 items receives 10 text deltas; only the last item re-renders per delta.
• Test: a document action registered after a document card is shown appears on that card without any other state change; removing it makes it disappear.
• Existing ChatPanel*.test.tsx and src/components/chat/* tests pass; their count does not decrease.
• npm run lint, npx vitest run, npm run build and npm run test:e2e pass.

Technical Notes

• Frontend only; no filter, risk, redaction or credential logic. The confirmation card of a proposed action must re-render when its decision or risk assessment changes (covered by existing tests).
• New code comments in English.

Spec impact: none

Related Work

• Chat: extract useChatSend and the action-decision handlers; ChatPanel within the size limits #847, Extension registry: make document actions and first-run notice extensions reactive #884