UX: Pin chat window to bottom when scrolling through history

When reviewing long conversation history, scrolling causes the view to jump around, losing context. Request: pin/anchor the chat input area to the bottom of the viewport so users can scroll through history without losing their place.

Current behavior: Scrolling through history can cause disorienting jumps
Expected behavior: Chat input stays fixed at bottom, history scrolls independently above it

This would improve UX for long sessions where reviewing prior context is needed.