feat: add Question Bookmarks & Personal Notes

[karan-chaos]

Description:
Implement a Question Bookmarks system that allows users to save important questions, add personal notes, organize bookmarks, and quickly find them later.

Key Features
Bookmark questions with category and difficulty
Add custom tags and personal notes
Star/favorite important bookmarks
Search across questions, notes, and answers
Filter by category, difficulty, and starred status
Paginated bookmark collection
Inline note editing
Expandable bookmark details
Bookmark statistics and summary
Authenticated REST API with CRUD support
Zod validation for bookmark creation and updates
React-based bookmark management interface

[github-actions]

Thanks for creating this issue!

If you'd like to work on it, simply comment:

/claim

The first person to claim the issue will be assigned automatically.

Happy contributing!

[karan-chaos]

/claim

[github-actions]

✅ @karan-chaos has successfully claimed this issue!

You've been assigned. Happy coding!

Tip: Please open a Pull Request linked to this issue when you're ready.
Add Closes #2272 in your PR description.