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/settings/settings-sidebar.tsx

@@ -58,19 +58,6 @@ export function SettingsSidebar() {
         >
           {t('categoryAccount')}
         </Link>
-        <Link
-          href="/settings/general/smtp"
-          className={`flex items-center rounded-md px-2 py-1.5 text-sm transition-colors ${
-            isActive('/settings/general/smtp')
-              ? 'bg-sidebar-accent text-sidebar-accent-foreground font-medium'
-              : 'text-sidebar-foreground hover:bg-muted'
-          }`}
-          aria-current={
-            isActive('/settings/general/smtp') ? 'page' : undefined
-          }
-        >
-          {t('categorySmtp')}
-        </Link>
       </nav>
 
       {/* Dashboard category — existing items unchanged */}