🎮 Add landing page with hero selection (Story 1.5)

Landing page with animated tagline, dual CTA (adventure/express), and
HeroSelector component (3 heroes: Recruiter, Client, Developer) with full
keyboard accessibility (radiogroup, arrow nav, Enter confirm). Staggered
CSS animations respecting prefers-reduced-motion. Bilingual FR/EN.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/implementation-artifacts/sprint-status.yaml

@@ -48,7 +48,7 @@ development_status:
   1-2-base-donnees-migrations-initiales: review
   1-3-systeme-i18n-frontend-api-bilingue: review
   1-4-layouts-routing-transitions-page: review
-  1-5-landing-page-choix-heros: ready-for-dev
+  1-5-landing-page-choix-heros: review
   1-6-store-pinia-progression-bandeau-rgpd: ready-for-dev
   1-7-page-resume-express-mode-presse: ready-for-dev
   epic-1-retrospective: optional