fix: briefing discuss button — full article content + scroll + optimistic UI

Three bugs in discussArticle():
- Scroll selector was '.briefing-chat' (doesn't exist) → '.briefing-center';
  the panel never scrolled into view so the response was invisible until refresh
- Only 300-char snippet was sent to the LLM; now passes the full stored
  content (up to 2000 chars) from the backend
- User message wasn't shown until streaming ended; now added optimistically
  to messages[] immediately on click so it appears straight away

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/routes/briefing.py

@@ -388,6 +388,7 @@ async def list_news():
             "title": r["title"],
             "url": r["url"],
             "snippet": (r["content"] or "")[:300],
+            "content": r["content"] or "",
             "published_at": r["published_at"].isoformat() if r["published_at"] else None,
             "topics": r["topics"] or [],
             "source": r["feed_title"],