Added custom subject input with automatic subject creation

[ShreyaDhomane]

#Related Issue
Closes #413

Summary

Improved the task creation experience by replacing the subject dropdown with a searchable text input and enabling automatic custom subject creation while adding tasks.

Changes Made

• Replaced subject <select> dropdown with text input + datalist suggestions.
• Added searchable subject suggestions using existing subjects.
• Implemented automatic subject creation if a typed subject does not already exist.
• Updated task save logic to support dynamically created subjects.
• Improved validation and task creation workflow.

Testing

• Tested task creation with existing subjects.
• Tested task creation with completely new custom subjects.
• Verified new subjects are added successfully and tasks are saved correctly.
• Tested modal opening/closing behavior.
• Checked for console errors after implementation.

Screenshots

Add screenshots of:

• New task modal
• Custom subject input
• Successfully created task with new subject

Checklist

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