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']
         }
     ];
 

frontend/src/routes/profile/account/+page.svelte

@@ -10,6 +10,7 @@
     import SlidersHorizontal from '@lucide/svelte/icons/sliders-horizontal';
     import KeyRound from '@lucide/svelte/icons/key-round';
     import LogOut from '@lucide/svelte/icons/log-out';
+    import Shield from '@lucide/svelte/icons/shield';
     import ChevronRight from '@lucide/svelte/icons/chevron-right';
 
     let currentPassword = $state('');
@@ -193,6 +194,18 @@
             </button>
         </div>
 
+        {#if session.user.is_admin}
+            <!-- Admin is reachable from the desktop header; the mobile chrome
+                 has no Admin tab, so the Account hub is its entry point. -->
+            <div class="row-group">
+                <a class="row" href="/admin" data-testid="account-row-admin">
+                    <Shield size={18} aria-hidden="true" />
+                    <span class="row-label">Admin</span>
+                    <ChevronRight size={16} aria-hidden="true" class="row-chev" />
+                </a>
+            </div>
+        {/if}
+
         <div class="row-group">
             <button
                 type="button"