feat: add ColorScheme caching and in-game theme switching

- Cache ColorScheme in ThemeManager to avoid repeated JSON parsing
- Add in-game theme switching methods:
  * set_theme_by_name() for string-based theme selection
  * get_current_theme_name() for current theme retrieval
  * get_available_themes() for theme listing
- Load language colors from separate JSON files based on theme type
- Update Colors API with new consolidated color methods
- Remove fallback compatibility for old color names

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: unhappychoice