feat: add edit and delete functionality for subjects

[aishwary-vansh]

Related Issue

Closes #1000

Summary

Implemented full CRUD support for Subjects, allowing users to edit and delete subjects directly from the sidebar. Also improved subject-related modals for a more consistent UI experience.

Changes Made

Backend

• Added PUT /api/subjects/:id to update subject names and colors.
• Added DELETE /api/subjects/:id with safe reassignment of related tasks to a default category.

Frontend

• Added updateSubject() and deleteSubject() methods in store.js.
• Added Edit (✏️) and Delete (🗑️) actions for subjects in the sidebar.
• Implemented optimistic UI updates for a smoother user experience.

UI Improvements

• Redesigned New Subject, Edit Subject, and Delete Confirmation modals.
• Fixed theme-related styling issues in Light/Dark mode.
• Improved subject color picker rendering and consistency.

Testing

• Tested subject creation, editing, and deletion through the UI.
• Verified tasks remain intact after subject deletion.
• Verified UI updates correctly and recovers from API failures.
• Tested modal appearance in both Light and Dark modes.

Screenshots

Checklist

• Code follows project style guidelines
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)

[aishwary-vansh]

@Charushi06

I have successfully implemented the Edit and Delete Subject functionality in the sidebar and tested it locally. The feature is working as intended, and I have attached the relevant screenshots in the PR.

Could you please review the changes and, if they meet the project requirements, consider merging the PR under GSSoC 2026?