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/src/routes/+layout.svelte

@@ -82,7 +82,9 @@
             label: 'Account',
             href: '/profile/account',
             icon: User,
-            match: ['/profile', '/profile/preferences']
+            // `/admin` keeps the Account tab highlighted while an admin is in
+            // the admin area — its only mobile entry point is the Account hub.
+            match: ['/profile', '/profile/preferences', '/admin']
         }
     ];