feat(nav): give admins a mobile entry point to the admin area

The admin area was reachable only from the desktop header's is_admin link;
on mobile there was no tab or row, so admins had to type the URL and no tab
highlighted. Add a conditional Admin row to the mobile Account hub and
`/admin` to the Account tab's match prefixes, so the tab stays highlighted
while an admin is in the admin area.

e2e: an admin session shows an Admin row linking to /admin; a non-admin
never sees it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.88.0",
+  "version": "0.89.0",
   "private": true,
   "type": "module",
   "scripts": {