feat: Replace abstract wireframes with health economics graphs in hero background

Replace 8 mathematical shapes (torus, icosahedron, etc.) with recognizable
health economics visualizations: Kaplan-Meier curve, cost-effectiveness plane,
Markov state diagram, decision tree, tornado diagram, bar chart, bell curve,
and network meta-analysis graph. Reduces rotation speed for legibility.

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

Author: danribes