[P0.21] Social proof on landing (stars, downloads, showcase)

[EmersonBraun]

User Story

Como visitante, quero ver social proof na landing (GitHub stars, downloads npm, used by, testimonials quando existirem, showcase).

Context

Derivada do Phase 0 PRD #211 — story #21.
Plano de execução: PHASE-0-EXECUTION-PLAN.md

Definition of Done

• Implementation
• Testes/validação
• Documentation updated
• Changeset (se aplicável)
• PR aprovado e merged