Add full landing page: all 10 sections with interactive React + KaTeX

Replaces placeholder with a complete research-grade landing page including:
- Animated network hero with causal flow diagram
- Interactive recourse slider demo (loan decision with live recourse)
- Three Pillars, Methods Library with tag filters
- Benchmark section with KaTeX formulas and animated progress bars
- Interactive sandbox with SVG decision boundary visualization
- Open Problems, Learn, and Footer with CHARM Lab attribution

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: amirhk