refactor(admin): move SMTP settings from user settings to admin area

SMTP configuration is an admin concern, not a per-user setting. Removed
it from the settings sidebar and relocated to /admin/smtp with a link in
the admin sidebar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/components/admin/admin-sidebar.tsx

@@ -60,6 +60,17 @@ export function AdminSidebar() {
         </svg>
       ),
     },
+    {
+      label: t('admin.smtp'),
+      href: '/admin/smtp',
+      show: true,
+      icon: (
+        <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+          <rect width="20" height="16" x="2" y="4" rx="2" />
+          <path d="m22 7-8.97 5.7a1.94 1.94 0 0 1-2.06 0L2 7" />
+        </svg>
+      ),
+    },
   ];
 
   return (