Issue for this PR

No related issue

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Makes the session chat scrollbar visible, white, and easier to interact with in the terminal TUI:

1. Scrollbar shown by default — changed KV default from false to true
2. White thumb — changed foregroundColor from theme.border (dim gray) to theme.text (white) for clear visibility on dark backgrounds
3. 2x taller thumb — overrides getVirtualThumbSize() on the slider to double the draggable thumb height for easier mouse interaction
4. Smooth momentum scrolling — replaced CustomSpeedScroll(3) with MacOSScrollAccel for natural acceleration-based scrolling
   The scrollbar can still be toggled off via the session.toggle.scrollbar command.

How did you verify your code works?

Tested locally by running bun run dev and verifying the scrollbar appears on the right side of the session chat.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR