- Add postMessageAndStream and savePartialMessage to chatApi (fetch ReadableStream for POST SSE)
- Create useStreamingChat hook with startStream, stop, streamingContent, isStreaming
- startTransition wraps token updates to avoid blocking user input (PERF-02)
- AbortController used for stop functionality (CHAT-12)
- stop() saves partial content with [stopped] suffix to DB
- Add @testing-library/react devDependency to enable renderHook testing
- 5 passing unit tests: token accumulation, lifecycle, stop/abort, error, null guard