Added optionnal online storage and cross-client sync

tests/sync-server.js

@@ -0,0 +1,5 @@
+// Isolated, disposable API for browser tests. Never opens the production DB.
+import { createSyncServer } from '../server/app.js';
+const server = createSyncServer({ maxVaults: 100 });
+server.listen(3001, '127.0.0.1');
+for (const signal of ['SIGTERM', 'SIGINT']) process.on(signal, () => server.close());