fix: await useAuthEmulator before completing Yust.initialize

lib/src/services/yust_auth_service_dart.dart

@@ -20,6 +20,11 @@ class YustAuthService {
   final Yust _yust;
   final ServiceAccountCredentials? _credentials;
 
+  /// Resolves once the service is ready. On Dart there is nothing async to
+  /// wait for, so this is always already completed — kept here to match the
+  /// Flutter-side API used in [Yust.initialize].
+  final Future<void> ready = Future.value();
+
   /// The Firebase Auth ID (uid) used to generate tokens for backend-to-backend
   /// API calls. When set, [getJWTToken] signs in as this user via a custom
   /// token exchange instead of throwing [UnsupportedError].

lib/src/services/yust_auth_service_flutter.dart

@@ -15,19 +15,25 @@ class YustAuthService {
   late final FirebaseAuth _fireAuth;
   late final Yust _yust;
 
+  /// Resolves once any emulator wiring has been applied. Callers that perform
+  /// auth operations during initialization must await this — `useAuthEmulator`
+  /// returns a Future that the constructor cannot await, and on web the
+  /// underlying JS `connectAuthEmulator` does not take effect synchronously.
+  final Future<void> ready;
+
   YustAuthService(
     Yust yust, {
     String? emulatorAddress,
     ServiceAccountCredentials? credentials,
     String? backendAuthId,
-  }) : _fireAuth = FirebaseAuth.instance,
-       _yust = yust {
-    if (emulatorAddress != null) {
-      _fireAuth.useAuthEmulator(emulatorAddress, 9099);
-    }
+  }) : ready = emulatorAddress != null
+           ? FirebaseAuth.instance.useAuthEmulator(emulatorAddress, 9099)
+           : Future.value() {
+    _fireAuth = FirebaseAuth.instance;
+    _yust = yust;
   }
 
-  YustAuthService.mocked(Yust yust) {
+  YustAuthService.mocked(Yust yust) : ready = Future.value() {
     throw UnsupportedError('Not supported in Flutter Environment');
   }
 

lib/src/services/yust_auth_service_mocked.dart

@@ -7,6 +7,9 @@ import 'yust_auth_service.dart';
 class YustAuthServiceMocked implements YustAuthService {
   final Yust _yust;
 
+  @override
+  final Future<void> ready = Future.value();
+
   YustAuthServiceMocked(Yust yust) : _yust = yust;
 
   @override

lib/src/yust.dart

@@ -190,6 +190,9 @@ class Yust {
       credentials: credentials,
       backendAuthId: backendAuthId,
     );
+    // On web, `useAuthEmulator` is async — sign-ins started before it
+    // resolves will hit production endpoints.
+    await Yust.authService.ready;
     Yust.fileService = YustFileService(
       authClient: authClient,
       emulatorAddress: emulatorAddress,