Add useSSE hook that connects to /api/threads/:id/live via EventSource.
Thread detail page now shows records in real-time with auto-scroll.
- useSSE hook: EventSource connection, record accumulation, auto-reconnect
with exponential backoff, cleanup on unmount
- Thread detail: Live badge, SSE-first with fetch fallback, smooth scroll
- Records clear on reconnect (server replays full file)
Closes#131, testing verified per #133
Refs: #118
xingyue
a44f1f34a8