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

lib/app.dart

@@ -14,6 +14,7 @@ import 'l10n/gen/app_localizations.dart';
 import 'modelos/alarma_musical.dart';
 import 'pantallas/pantalla_alarmas.dart';
 import 'pantallas/pantalla_alarma_sonando.dart';
+import 'pantallas/pantalla_bienvenida.dart';
 import 'pantallas/pantalla_inicio.dart';
 import 'pantallas/pantalla_buscar.dart';
 import 'pantallas/pantalla_favoritos.dart';
@@ -108,7 +109,10 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
   EstadoRadio? _estadoSuscrito;
   bool _alarmaInicialProcesada = false;
   bool _alarmaSonandoActiva = false;
-  bool _onboardingInicialSolicitado = false;
+  // WU17b: renamed from `_onboardingInicialSolicitado` — this single guard
+  // now covers the whole first-launch sequence (welcome screen, then the
+  // pre-existing what's-new dialog), not only the dialog.
+  bool _flujoPrimerLanzamientoSolicitado = false;
   String? _alarmaSonandoId;
   Locale? _localeAlarmasConfigurado;
 
@@ -191,9 +195,9 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
       _alarmaInicialProcesada = true;
       unawaited(_procesarAlarmaInicial(alarmas));
     }
-    if (!_onboardingInicialSolicitado) {
-      _onboardingInicialSolicitado = true;
-      unawaited(_mostrarOnboardingInicial());
+    if (!_flujoPrimerLanzamientoSolicitado) {
+      _flujoPrimerLanzamientoSolicitado = true;
+      unawaited(_mostrarFlujoPrimerLanzamiento());
     }
   }
 
@@ -278,6 +282,19 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
     }
   }
 
+  // WU17b: runs the welcome screen's once-ever check BEFORE the recurring
+  // what's-new dialog, so the two never show at the same time. The welcome
+  // screen (PantallaBienvenida) is the genuine first-run surface; the
+  // pre-existing PluriOnboardingDialog is an unrelated "what's new"/help
+  // modal that keeps its own independent per-version due-or-not logic,
+  // completely unchanged by this sequencing.
+  Future<void> _mostrarFlujoPrimerLanzamiento() async {
+    if (mounted) {
+      await PantallaBienvenida.mostrarSiProcede(context);
+    }
+    await _mostrarOnboardingInicial();
+  }
+
   Future<void> _mostrarOnboardingInicial() async {
     await Future<void>.delayed(const Duration(milliseconds: 900));
     if (!mounted || _alarmaSonandoActiva) return;

lib/pantallas/pantalla_bienvenida.dart

@@ -3,6 +3,7 @@ import 'package:provider/provider.dart';
 
 import '../estado/estado_navegacion.dart';
 import '../l10n/gen/app_localizations.dart';
+import '../servicios/servicio_bienvenida.dart';
 import '../tema/pluriwave_theme.dart';
 
 /// WU17: first-run welcome surface (`onboarding-welcome` spec, mockup
@@ -14,13 +15,31 @@ import '../tema/pluriwave_theme.dart';
 ///
 /// A full-screen ROUTE, not a modal `Dialog` — deliberately distinct from
 /// the pre-existing `PluriOnboardingDialog` (an unrelated "what's new"
-/// help-content modal already shown from `app.dart`'s launch flow). This
-/// work unit's own task list and verify command scope to this screen in
-/// isolation; wiring it into the real first-launch flow (i.e. deciding
-/// when `app.dart`/`main.dart` should push it) is not part of WU17.
+/// help-content modal already shown from `app.dart`'s launch flow). Both
+/// surfaces coexist: [mostrarSiProcede] (WU17b) is what actually wires this
+/// screen into the genuine first-launch flow, called from `app.dart` BEFORE
+/// `PluriOnboardingDialog.mostrarSiProcede` on every cold start, so the two
+/// never race — the once-ever welcome resolves first, then the recurring
+/// what's-new dialog runs its own unrelated per-version check exactly as
+/// before.
 class PantallaBienvenida extends StatelessWidget {
   const PantallaBienvenida({super.key});
 
+  static final ServicioBienvenida _servicio = ServicioBienvenida();
+
+  /// WU17b: shows this screen once, on the genuine first launch, then never
+  /// again. Mirrors `PluriOnboardingDialog.mostrarSiProcede`'s shape
+  /// (check-then-show-then-mark-seen) so both first-launch surfaces share
+  /// the same call convention from `app.dart`.
+  static Future<void> mostrarSiProcede(BuildContext context) async {
+    if (!await _servicio.debeMostrarBienvenida()) return;
+    if (!context.mounted) return;
+    await Navigator.of(
+      context,
+    ).push(MaterialPageRoute<void>(builder: (_) => const PantallaBienvenida()));
+    await _servicio.marcarBienvenidaVista();
+  }
+
   @override
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);

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);
+  }
+}