Add registration validation and more activities

[siliak]

This pull request enhances the extracurricular activities feature by adding new activities, improving activity signup validation, and significantly upgrading the user interface to display participant lists and modernize the layout. The changes span backend data, API logic, frontend rendering, and styling.

Backend changes:

• Added several new activities (sports, arts, intellectual) to the activities data structure in src/app.py, each with detailed information and initial participants.
• Updated the signup_for_activity function in src/app.py to prevent students from signing up for the same activity more than once by raising an error if already registered.

Frontend logic and display:

• Refactored the frontend activity rendering in src/static/app.js by introducing a createActivityElement function that displays each activity along with a participant list, showing either current participants or a placeholder if none exist. [1] [2]

Styling and layout improvements:

• Updated the activities list container in src/static/index.html to use a grid layout for better visual organization.
• Enhanced the CSS in src/static/styles.css to support the new grid layout, improve the appearance of activity cards, and style the participants section and list for clarity and modern aesthetics. [1] [2]

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