feat(tui): add appearance setting with dark/light/system modes

[nguyenphutrong]

Summary

Adds an appearance config option to control the TUI color scheme with three modes:

• dark: Always use dark variant of theme
• light: Always use light variant of theme
• system: Auto-detect from terminal background color (polls every 5s)

Changes

• Added appearance field to config schema (dark | light | system)
• Added appearance selector UI in theme dialog with ←/→ keyboard navigation
• Implemented theme mode detection (analyzes theme JSON to determine dark-only/light-only/both support)
• Auto-disables unsupported modes for single-mode themes (e.g., aura, catppuccin-macchiato are dark-only)
• Real-time terminal background detection using renderer.getPalette() for cross-platform support
• Regenerated SDK types to include appearance field

Usage

Via Config

{
  "appearance": "system"
}

Via TUI

1. Press Ctrl+X T to open theme dialog
2. Use ←/→ to switch between System/Light/Dark modes
3. Use ↑/↓ to browse themes

Screenshots

The appearance selector appears at the top of the theme dialog. Unsupported modes are dimmed and cannot be selected.

Testing

• Tested with dark-only themes (aura, ayu, catppuccin-macchiato, nightowl)
• Tested with dual-mode themes (flexoki, github, opencode, tokyonight)
• Verified real-time system mode polling works across terminal theme changes