feat: add landing page hero section with animated background

- Create ColorWaveBackground canvas component with animated gradient blobs
- Create HeroSection with ShieldAI branding, headline, CTAs, and trust signals
- Compose landing page in index.tsx route with background + hero
- Add unit tests for HeroSection (13 tests) and ColorWaveBackground (8 tests)
- Background respects prefers-reduced-motion accessibility preference
- Responsive layout with mobile-first text sizing and stacked buttons

web/src/components/landing/index.ts

@@ -0,0 +1,2 @@
+export { default as ColorWaveBackground } from "./ColorWaveBackground";
+export { default as HeroSection } from "./HeroSection";