fix(webview): keep chat pinned to bottom during streaming via ResizeObserver

Extracted from #39. Auto-scroll currently only fires when the message count changes, so token growth inside an existing message (or content expanding) never re-pins the view.

Add an optional `contentRef` to `useAutoScroll` and a ResizeObserver that re-pins `scrollTop` on content height changes while streaming, unless the user has scrolled up. Reference implementation: `useAutoScroll.ts` / `MessageList.tsx` changes in #39.

**Size**: S | **Depends on**: nothing | **Tier**: 0 (quick win)