docs: expand WEB_APP_GUIDE with interactive animations and advanced patterns

- Enhanced welcome section with CSS animations (gradient, float, fade-in effects)
- Added 2,000+ lines of comprehensive documentation
- Included advanced architecture patterns (StateManager, ComponentRegistry, APIClient)
- Added comprehensive folder structure walkthrough with file descriptions
- Expanded developer guides (performance profiling, troubleshooting, DevTools)
- Added testing patterns, error handling, and security best practices
- Included analytics, i18n, gesture handling, and utility functions
- Enhanced accessibility documentation with code examples
- Added responsive design patterns and CSS techniques
- Total: 2,894 lines, 82.5 KB - Exceeds target of 2,500-3,000 lines

Author: mahi-8758