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/borrows/hooks.ts

@@ -1,6 +1,7 @@
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query"
 import { getAllMyBorrows, returnBookForMe } from "@/api/borrows"
 import { toast } from "sonner"
+import { getErrorMessage } from "@/lib/errors"
 
 export function useMyBorrows() {
   return useQuery({
@@ -18,7 +19,7 @@ export function useReturnBook() {
       toast.success("归还成功")
     },
     onError: (err) => {
-      toast.error(err instanceof Error ? err.message : "归还失败")
+      toast.error(getErrorMessage(err, "归还失败"))
     },
   })
 }