feat(tui): enable scrollbar on hover for chat history

[Marve10s]

Summary

Shows the vertical scrollbar on hover in the chat history column when content is scrollable.

Changes

• Scrollbar appears only when hovering over the chat history area
• Only visible when content actually overflows (scrollHeight > viewport height)
• Respects existing toggle setting via command palette

Why

1. Difficult to gauge conversation length/position in long sessions without visual indicator
2. Related to scroll UX issues reported in #6895 and #6901 - having a visible scrollbar helps users understand scroll state

Notes

This builds on the existing scrollbar_toggle feature (which was disabled by default). The scrollbar visibility logic was already implemented - this PR just enables it by default with hover behavior.