feat(email): Add high-priority email templates

Commit notes

Created 5 transactional email templates using React Email + Tailwind:

1. Purchase Receipt (buyer) - Order details with pack info - Amount, date, order ID - Download button and library link

2. Sale Notification (creator) - Earnings highlight with gradient card - Sale breakdown (amount, fee, earnings) - Balance summary with dashboard link

3. Workspace Invitation - Personal message from inviter - Role badge with description - Accept button with expiration notice

4. Subscription Confirmation - Plan features checklist - Storage limit highlight - Billing details and next payment date

5. Storage Warning - Visual progress bar - Conditional messaging (80% vs 100%) - Upgrade and library management options

All templates: - Use consistent EmailLayout component - Support Tailwind CSS styling - Include preview props for React Email playground - Export typed helper functions for easy use

Co-authored-by: armin.naimi <[email redacted]>