Smooth token streaming

[charlesvien]

https://upstash.com/blog/smooth-streaming

Make sure it still scrolls to the bottom / auto-fixes to the bottom as it streams

[archievi]

Opened #2685 for this. It adds a useSmoothText hook that reveals streamed tokens at a steady rate via requestAnimationFrame (per the Upstash smooth-streaming approach), so bursty chunks read as smooth typing. History/completed messages render instantly (no typewriter replay), reduced-motion snaps, and auto-scroll-to-bottom is preserved. Happy to adjust the reveal rate or approach if you have a preference.