feat(admin): add book borrowing dialog and root redirect

- Add admin book borrowing dialog with user selection
- Add useAdminBorrowBook mutation with success/error toasts
- Fix sidebar z-index layering on admin layout
- Add RootRedirect component that routes based on admin role

src/features/books/hooks.ts

@@ -2,6 +2,7 @@ import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query"
 import { getAllBooks } from "@/api/books"
 import { borrowBookForMe } from "@/api/borrows"
 import { toast } from "sonner"
+import { getErrorMessage } from "@/lib/errors"
 
 export function useBooks() {
   return useQuery({
@@ -21,7 +22,7 @@ export function useBorrowBook() {
       toast.success("借阅成功")
     },
     onError: (err) => {
-      toast.error(err instanceof Error ? err.message : "借阅失败")
+      toast.error(getErrorMessage(err, "借阅失败"))
     },
   })
 }