NO_FLICKER mode: slow mouse scroll and no usable scrollbar on Windows

Description

With CLAUDE_CODE_NO_FLICKER=1 enabled, scrolling through long responses is painfully slow and the terminal's native scrollbar is not accessible.

The NO_FLICKER renderer uses the alternate screen buffer, which disables the terminal's native scrollbar. The app handles scrolling internally, but:

1. Mouse scroll speed is very slow — each scroll tick moves only a small amount, making it tedious to navigate long outputs
2. No scrollbar available — the terminal's native scrollbar is inaccessible (expected with alt screen), but there's no in-app scrollbar or fast-scroll mechanism to compensate

Both issues disappear when CLAUDE_CODE_NO_FLICKER is disabled, since the normal renderer uses the primary screen buffer where the terminal's native scrollbar and scroll speed work as expected.

Steps to Reproduce

1. Set "CLAUDE_CODE_NO_FLICKER": "1" in env in ~/.claude/settings.json
2. Start a Claude Code session
3. Generate a long response (e.g., ask Claude to explain a complex topic in detail)
4. Try to scroll up through the output with the mouse wheel — scroll speed is very slow
5. Try to use the terminal scrollbar — it's not accessible

Expected Behavior

Scrolling through long outputs should be fast enough to be usable, either by:

• Increasing the number of lines per scroll tick
• Providing an in-app scrollbar or scroll acceleration
• Supporting Page Up/Page Down for large jumps

Environment

• OS: Windows 11 Pro (10.0.26220)
• Terminal: Windows Terminal
• Shell: Git Bash