feat(ui): add scroll support with PageUp/PageDown and mouse/trackpad scrolling

[R3LAMP4GO]

Summary

Adds viewport-based scrolling to the interactive TUI.

New files

• useContentScroll hook — manages viewport offset, scroll up/down/toBottom
• useMouseScroll hook — SGR mouse protocol for trackpad/wheel events
• ScrollIndicator component — visual scroll position indicator

Changes

• App.tsx — integrates scroll viewport, PageUp/PageDown key bindings, mouse scroll handlers
• ActivityIndicator, InputArea, Markdown, SelectList — compatibility updates for scroll state
• useAgentLoop, render.ts — scroll state management wiring

How it works

• PageUp/PageDown scrolls by half-viewport
• Mouse wheel/trackpad scrolls 3 lines at a time
• SGR mouse tracking only activates when content exceeds viewport (preserves native terminal scrollback)
• Auto-scrolls to bottom when agent completes a turn

[MontaEllis8]

✅ Code Review 通过

代码审查通过，建议合并。