add input for number of copies
add backend for number of copies
validation for add book form
validation for add book form
test backend book with null title
Backend validation for books.
refactored get/set borrower to lend methods
Book can be borrowed by multiple users (if enough copies are available).
E2E test for borrowing a book with one copy.
Update BookTo for multiple borrowers.
Logic for showing borrow/return buttons.
Test author.
Test author.
Test author.
Move bookBorrowedByCurrentUser logic from client to server.
More tests for BookToMapper.
mapper test for no borrowers
Remove userHasBook logic from client.
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