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/admin/AdminBorrowsPage.tsx

@@ -20,6 +20,7 @@ import { useAdminBorrows, useSearchBorrows, useAdminReturnBook } from "./hooks"
 import { usePlatform } from "@/hooks/usePlatform"
 import { useIsMobile } from "@/hooks/useIsMobile"
 import { formatDateTime } from "@/lib/formatters"
+import { getErrorMessage } from "@/lib/errors"
 import type { BorrowInfoVo } from "@/types/api"
 
 export default function AdminBorrowsPage() {
@@ -47,7 +48,7 @@ export default function AdminBorrowsPage() {
   if (error) {
     return (
       <div className="rounded-lg border border-destructive/50 bg-destructive/10 px-4 py-3 text-sm text-destructive">
-        加载失败：{error instanceof Error ? error.message : "未知错误"}
+        加载失败：{getErrorMessage(error)}
       </div>
     )
   }