docs: add comprehensive visual style guide
Add VISUAL_STYLE_GUIDE.md to standardize UI development:
- Design principles and keywords
- Complete color system (CSS variables, semantic usage)
- Typography hierarchy and patterns
- Spacing, layout, and container specs
- Border radius and shadow system
- Component specs (Button, Input, Card, Dialog, etc.)
- Interaction and animation guidelines
- Dark mode implementation rules
- Tool color palette and assignment
- Code organization standards
- Anti-patterns checklist
- Quick reference appendix
Update .gitignore to allow tracking the style guide.
雨霖铃
f8c48a3047