feat: add in-app admin navigation for whitelisted users

Detect admin access after login and expose a header button that opens /admin via client-side routing so the session stays unlocked when returning to the app.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/components/AdminHeaderButton.tsx

@@ -0,0 +1,23 @@
+import { useTranslation } from 'react-i18next'
+import { LayoutDashboard } from 'lucide-react'
+
+interface AdminHeaderButtonProps {
+  onClick: () => void
+}
+
+export default function AdminHeaderButton({ onClick }: AdminHeaderButtonProps) {
+  const { t } = useTranslation()
+
+  return (
+    <button
+      type="button"
+      className="btn-icon skipper-badge"
+      onClick={onClick}
+      title={t('nav.admin')}
+      aria-label={t('nav.admin')}
+    >
+      <LayoutDashboard size={18} aria-hidden="true" />
+      <span className="skipper-badge__name">{t('nav.admin')}</span>
+    </button>
+  )
+}