Improve student activity registration system

[Christine8605]

This pull request adds support for unregistering students from activities, improves the display of participants in the UI, and introduces comprehensive backend tests to ensure the reliability of signup and unregister features. It also expands the list of available activities.

Backend functionality and validation:

• Added a DELETE /activities/{activity_name}/signup endpoint to allow students to unregister from activities, with appropriate error handling for missing activities or students not registered.
• Improved signup validation to prevent duplicate registrations by raising an error if a student is already signed up.

Frontend UI improvements:

• Updated the activities list to display all participants for each activity, including an "unregister" (delete) button next to each participant. [1] [2]
• Implemented frontend logic to handle unregistering participants via the new API endpoint, refreshing the activities list upon success. [1] [2]

Testing:

• Added a comprehensive test suite (tests/test_app.py) covering activity listing, signup, and unregister endpoints, including edge cases and isolation between tests.

Data updates:

• Added several new activities with sample participants to the in-memory data store, such as Basketball Team, Tennis Club, Art Class, Music Band, Debate Club, and Science Club.

[Copilot]

Pull request overview

Adds unregister support for student activity signups end-to-end (backend API + frontend UI) and introduces backend tests to cover core signup/unregister flows.

Changes:

• Backend: add DELETE /activities/{activity_name}/signup and prevent duplicate signups.
• Frontend: render participant lists per activity and allow unregistering via a delete button; refresh UI after changes.
• Testing/data: add pytest suite for activities endpoints and expand seeded activities.

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/app.py	Adds unregister endpoint, duplicate-signup validation, and expands seeded activities.
src/static/app.js	Renders participants + unregister buttons and triggers refresh after signup/unregister.
src/static/styles.css	Styles participant list and delete/unregister button.
tests/test_app.py	Adds pytest coverage for listing, signup, duplicate signup, and unregister scenarios with isolation.
requirements.txt	Adds pytest dependency and fixes line formatting for watchfiles.
tests/__init__.py	Marks tests as a package (no functional changes).

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[Copilot]

fetchActivities() repopulates the <select id="activity"> by appending options but never clears existing options. Now that fetchActivities() is called after signup/unregister, the dropdown will accumulate duplicate activity options on every refresh. Clear/reset the select (preserving the placeholder option) before appending new options, or rebuild options from scratch each fetch.