fix(bienvenida): wire the welcome screen into the first-launch flow

lib/servicios/servicio_bienvenida.dart

@@ -0,0 +1,32 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// WU17b: persists whether the first-run welcome screen (`PantallaBienvenida`,
+/// `onboarding-welcome` spec) has already been shown, so it renders once
+/// rather than on every launch.
+///
+/// Same injectable-`SharedPreferences`, versioned-key convention as
+/// `ServicioContenidoApp` (S3-R4) — but a plain one-time boolean flag, since
+/// this welcome screen is a single first-impression surface, not something
+/// that re-triggers per app version the way the "what's new" onboarding
+/// dialog does.
+class ServicioBienvenida {
+  ServicioBienvenida({SharedPreferences? prefs}) : _prefs = prefs;
+
+  static const _keyBienvenidaVista = 'pluri_bienvenida_vista_v1';
+
+  final SharedPreferences? _prefs;
+
+  /// Injected startup instance (S3-R4); getInstance() is only a fallback.
+  Future<SharedPreferences> _resolverPrefs() async =>
+      _prefs ?? SharedPreferences.getInstance();
+
+  Future<bool> debeMostrarBienvenida() async {
+    final prefs = await _resolverPrefs();
+    return !(prefs.getBool(_keyBienvenidaVista) ?? false);
+  }
+
+  Future<void> marcarBienvenidaVista() async {
+    final prefs = await _resolverPrefs();
+    await prefs.setBool(_keyBienvenidaVista, true);
+  }
+}