feat: Add multi-message email system with local testing support

🚀 What's Changed

This PR transforms the simple email script into a comprehensive multi-message email automation system for Django Girls Colombia workshops.

✨ Major Features Added

🎯 Multi-Message Support

• Support for 5 different email types:
  • certificate - Post-workshop certificates for attendees
  • accepted-participants - Acceptance notifications for participants
  • waitlist-participants - Waitlist notifications for participants
  • accepted-mentors - Acceptance notifications for mentors/guides
  • waitlist-mentors - Waitlist notifications for mentors

🧪 Local Testing Mode

• New --local flag generates HTML preview files instead of sending emails
• Perfect for testing templates and data before sending actual emails
• Preview files saved to output/ directory with descriptive names

📧 Professional Email Templates

• Complete set of responsive HTML email templates
• Consistent branding and styling across all message types
• Proper template variable system that safely handles CSS/JS braces

⚙️ Better Code Organization

• Proper argument parsing with argparse
• Organized configuration constants
• Separate functions for different email workflows
• Improved error handling and CSV validation
• Better template loading with safe variable replacement

🗂️ File Structure Improvements

• Moved data files to data/ directory
• Added comprehensive email templates in templates/
• Updated .gitignore to exclude output files
• Completely rewritten README with detailed usage instructions

🔧 Usage Examples

# Test certificate emails locally
python main.py --type certificate --local

# Send acceptance emails to mentors
python main.py --type accepted-mentors

# Test participant waitlist notifications
python main.py --type waitlist-participants --local

🛠️ Technical Changes

• Breaking Change: Command-line interface now requires --type parameter
• Template variables properly contextualized per message type
• Dynamic CSV file selection based on email type
• Improved Gmail API integration with better error handling
• Workshop dates updated to 2026

This upgrade provides a robust, testable, and maintainable email automation system that scales with Django Girls Colombia's workshop management needs.