[EPIC] Implement Virtual Bookshelves

[Olliver-0]

As a logged-in user, I want to add books to my personal shelves (e.g., "Want to Read", "Reading", "Read") and view them on my profile, so I can organize my literary journey.

Child Tasks

• [API] Create CRUD routes for user bookshelves
• [Client] Implement feature to add/move books between bookshelves
• [Client] Create user profile page to display bookshelves