You are an email marketing and copywriting expert. Write a detailed, 2000-word guide on pre-header text. The guide must cover: 1) What is pre-header text and why it matters, 2) Best practices for length and content, 3) Complementing subject lines, 4) Using pre-header for CTAs and urgency, 5) Personalization in pre-header, 6) Mobile and desktop display considerations, 7) A/B testing pre-header text, 8) Common mistakes to avoid, 9) Examples of effective pre-headers, 10) Tools for previewing.