Improve student activity registration system

[framirez07]

Summary

This PR enhances a student activity registration system for Mergington High School by adding comprehensive unregister functionality, expanding the activity database with six new activities, improving the frontend UI with participant visibility and management, and introducing a complete test suite covering all API endpoints.

Key Changes by Component

Backend (src/app.py):

• Added 6 new student activities: Soccer Team, Swimming Club, Art Club, Drama Club, Math Team, and Science Olympiad
• Added duplicate signup prevention validation
• Introduced new DELETE /activities/{activity_name}/unregister endpoint to remove students from activities

Frontend (src/static/):

• app.js: Displays participant lists in activity cards with inline delete buttons; added event handlers for unregistering participants with error handling
• styles.css: Added styling for participants section, participant list items, and delete buttons with hover effects

Testing (tests/):

• Created comprehensive test suite with 31 test cases organized into 3 test classes
• Tests cover GET activities, successful/duplicate signups, unsuccessful signups, and unregister operations
• Includes pytest fixtures for activity state management and test client setup

Dependencies:

• Added pytest to requirements.txt