Add a visible scrollbar to the TUI for fine-grained scroll navigation

What

The new flicker-free rendering is great, but there's no visible scrollbar in the terminal UI. PgUp/PgDn work for paging, but there's no visual indicator of scroll position or content length, and no way to fine-tune navigation the way a scrollbar allows.

Why it matters

• Position awareness — a scrollbar shows where you are in a long transcript and how much content is above/below.
• Fine navigation — PgUp/PgDn jump in fixed increments; a scrollbar (and/or drag) lets you land precisely.

Request

Add a visible scrollbar to the conversation/transcript view, ideally alongside the existing keyboard paging.

Environment

• Claude Code: 2.1.152
• OS: Windows 11
• Terminal: (PowerShell-based session)