feat: add optional scrollbar to the session chat

[franlol]

Add toggleable scrollbar to session chat window,

Summary

• Add a vertical scrollbar to the chat window that can be toggled on/off via o (Control+X O) or command palette
• Scrollbar visibility is persisted across sessions

Usage

• Keyboard: Control+X O to toggle scrollbar visibility
• Command palette: Search "Toggle session scrollbar"
• Setting persists across sessions