Add registration validation and more activities

[sunil-archt]

This pull request introduces new features and improvements to the activity management system, including the addition of new activities, enhanced participant management, and UI updates to display participant details. Below is a breakdown of the most significant changes:

New Activities Added

• Added six new activities categorized into sports, arts, and intellectual pursuits: "Soccer Team," "Basketball Club," "Art Club," "Drama Society," "Math Olympiad," and "Science Club." Each activity includes a description, schedule, maximum participant limit, and initial participants. (src/app.py, src/app.pyR41-R79)

Backend Enhancements

• Updated the signup_for_activity function to prevent duplicate signups by checking if a student is already enrolled in the selected activity. If so, an HTTP 400 error is raised. (src/app.py, src/app.pyR104-R108)

Frontend Improvements

• Enhanced the activity card UI to display a list of current participants. If no participants are signed up, a message stating "No participants yet" is shown. (src/static/app.js, src/static/app.jsR23-R35)

Styling Updates

• Added styles for the new participants section, including a top border, spacing adjustments, and distinct styles for participant lists and the "no participants" message to improve readability. (src/static/styles.css, src/static/styles.cssR77-R100)