refactor(dto): use nested DTOs for borrow info

- Extract book borrow info into BookBorrowDto
- Extract user borrow info into UserBorrowDto
- Update BorrowInfoDto to reference the new DTOs

src/main/kotlin/com/msksbr/bookmgr/dto/BorrowInfoDto.kt

@@ -1,13 +1,13 @@
 package com.msksbr.bookmgr.dto
 
-import com.msksbr.bookmgr.entity.Book
+import com.msksbr.bookmgr.dto.borrow.BookBorrowDto
+import com.msksbr.bookmgr.dto.borrow.UserBorrowDto
 import java.util.Date
 
 class BorrowInfoDto(
     val id: Long,
-    val book: Book,
-    val userId: Long,
-    val username: String,
+    val bookBorrowDto: BookBorrowDto,
+    val userBorrowDto: UserBorrowDto,
     val borrowTime: Date,
     val returnTime: Date?,
     val string: String

src/main/kotlin/com/msksbr/bookmgr/dto/borrow/BookBorrowDto.kt

@@ -0,0 +1,8 @@
+package com.msksbr.bookmgr.dto.borrow
+
+data class BookBorrowDto(
+    val id: Long,
+    val name: String,
+    val author: String,
+    val number: Int = 1
+)

src/main/kotlin/com/msksbr/bookmgr/dto/borrow/UserBorrowDto.kt

@@ -0,0 +1,7 @@
+package com.msksbr.bookmgr.dto.borrow
+
+data class UserBorrowDto(
+    val id: Long,
+    val username: String,
+    val role: String
+)