Format Contact CSV for Klaviyo Profile Import

Klaviyo's profile import parser is case-sensitive and reserves all properties prefixed with '$' as built-in profile attributes. Columns named 'Email Address', 'email', or 'E-mail' will not auto-map to the $email reserved property — Klaviyo requires the exact column header '$email' to recognize and populate the primary identifier. Similarly, $first_name and $last_name must use the dollar-sign prefix; without it, Klaviyo creates custom properties instead of populating the profile fields that personalization tags like {% person 'first_name' %} reference, causing emails to render with blank greetings. For SMS marketing imports, Klaviyo enforces strict E.164 international phone number formatting — the number must include a leading '+', country code, and subscriber number with no spaces, dashes, or parentheses. A value like '(415) 555-0123' triggers 'Invalid Phone Format' and the row is rejected. This workflow renames your columns to match Klaviyo's reserved property schema exactly, strips and reformats phone numbers to E.164 (e.g., +14155550123), and validates email syntax before upload — all processed locally in your browser so subscriber PII never leaves your machine.