frontend: extract shared hero-card from --surface-hero (de-dup beta/sign-in modal)

[AndresL230]

[P1-E] Replace the duplicated inline 24px warm-gradient box in the beta modal (app/(public)/page.tsx) and components/SignInModal.tsx with one shared component backed by --surface-hero / --surface-hero-shadow. Deferred from token-unification Phase 1 (PR #286). Tracker: docs/superpowers/followups/2026-06-30-token-unification-followups.md · Audit: docs/frontend-rhythm-audit.md