fix: import UninstallMachineModal in AdminSidebar

The component was used but never imported, causing the sidebar to
crash when clicking the x button to remove a machine. The modal
rendered as undefined and React threw.

Author: reddy-sh

site/src/components/admin/AdminSidebar.jsx

@@ -1,4 +1,5 @@
 import { useRef, useState } from 'react'
+import { UninstallMachineModal } from '../board/enroll'
 import { Pill } from '../board/panels'
 import { Ic } from '../board/Rail'