feat: add PWA offline unlock support

webapp/src/lib/pwa.ts

@@ -0,0 +1,11 @@
+export function registerNodeWardenServiceWorker(): void {
+  if (typeof window === 'undefined') return;
+  if (!('serviceWorker' in navigator)) return;
+  if (import.meta.env.DEV) return;
+
+  window.addEventListener('load', () => {
+    void navigator.serviceWorker.register('/sw.js', { scope: '/' }).catch(() => {
+      // PWA support is progressive enhancement; the vault still works without it.
+    });
+  });
+}