chore(readme): rewrite README-PT-BR.md as comprehensive Dashboard Scripts guide

Rewrote the Portuguese README to serve as the official, authoritative guide for creating secure and maintainable Dashboard Scripts. Includes new sections on safe DOM anchoring, idempotent execution, low-fragility strategies, dashboard script vs app decision criteria, injection pipeline details, scope modes (global/account-specific), model constraints, editor/preview behavior, and robust utility functions for sidebar manipulation and account scoping.

Co-authored-by: Copilot <copilot@github.com>