fix(bienvenida): bottom-anchor content and add the blurred backdrop

Audit 14.2 (t4 lines 693/706): content was top-anchored in a plain
SingleChildScrollView; the prototype anchors it to the bottom with two
flexible spacers, so the CTA sits at the screen edge. Restructured as
a scrollable Expanded region (safe on short screens / large text
scale) with the CTA pinned below it, always at the bottom.

Audit 14.1 (t4 lines 687-689): added the full-bleed blurred banner
backdrop, reusing aurora_wave_banner.png (the same asset the deleted
PluriScreenHeader used) since this app has no bundled equivalent of
the prototype's mockup-only banner.jpg.

No monetization content touched (PRO pill, "14 días", pricing card,
free-version link all remain absent per the binding no-monetization
decision).

test/pantallas/pantalla_bienvenida_test.dart

@@ -39,7 +39,13 @@ void main() {
       expect(find.byType(AlertDialog), findsNothing);
       expect(find.byType(Scaffold), findsOneWidget);
 
-      expect(find.byType(Image), findsOneWidget, reason: 'the logo mark');
+      // Audit 14.1 adds a second Image (the blurred backdrop), so "the
+      // logo mark" is now asserted by key rather than "the only Image".
+      expect(
+        find.byKey(const ValueKey('welcome-logo')),
+        findsOneWidget,
+        reason: 'the logo mark',
+      );
       expect(find.text('Tu mundo, en directo'), findsOneWidget);
       expect(
         find.textContaining('53.412 emisoras de 238 países'),
@@ -77,9 +83,7 @@ void main() {
     expect(textoCompleto.contains('€'), isFalse);
     expect(textoCompleto.contains(r'$'), isFalse);
     expect(
-      RegExp(r'\d+\s*d[ií]as?\b', caseSensitive: false).hasMatch(
-        textoCompleto,
-      ),
+      RegExp(r'\d+\s*d[ií]as?\b', caseSensitive: false).hasMatch(textoCompleto),
       isFalse,
       reason: 'no day-count trial phrase (e.g. "14 días")',
     );
@@ -103,8 +107,7 @@ void main() {
     (tester) async {
       // Start on a DIFFERENT tab so a pass actually proves irA() ran,
       // rather than coasting on EstadoNavegacionRaiz's own escuchar default.
-      final navegacion = EstadoNavegacionRaiz()
-        ..irA(RaizPluriWave.ajustes);
+      final navegacion = EstadoNavegacionRaiz()..irA(RaizPluriWave.ajustes);
       addTearDown(navegacion.dispose);
       final navigatorKey = GlobalKey<NavigatorState>();
 
@@ -141,4 +144,51 @@ void main() {
       expect(find.text('home stand-in'), findsOneWidget);
     },
   );
+
+  testWidgets('visual fidelity (audit 14.1, proto t4 lines 687-689): a blurred '
+      'banner backdrop is present behind the content', (tester) async {
+    final navegacion = EstadoNavegacionRaiz();
+    addTearDown(navegacion.dispose);
+
+    await tester.pumpWidget(_app(navegacion: navegacion));
+    await tester.pumpAndSettle();
+
+    expect(
+      find.byKey(const ValueKey('welcome-background-art')),
+      findsOneWidget,
+      reason:
+          'the prototype draws a blurred, 40%-opacity banner image '
+          'full-bleed behind the content — this screen used to be a '
+          'plain Scaffold with no backdrop at all',
+    );
+  });
+
+  testWidgets(
+    'visual fidelity (audit 14.2, proto t4 lines 693/706): the primary CTA '
+    'is anchored to the bottom edge, not immediately after the bullets',
+    (tester) async {
+      final navegacion = EstadoNavegacionRaiz();
+      addTearDown(navegacion.dispose);
+      tester.view.physicalSize = const Size(1440, 3200);
+      tester.view.devicePixelRatio = 1.0;
+      addTearDown(tester.view.resetPhysicalSize);
+      addTearDown(tester.view.resetDevicePixelRatio);
+
+      await tester.pumpWidget(_app(navegacion: navegacion));
+      await tester.pumpAndSettle();
+
+      final alturaPantalla = tester.view.physicalSize.height;
+      final ctaAbajo = tester.getBottomLeft(find.byType(FilledButton)).dy;
+
+      expect(
+        ctaAbajo,
+        greaterThan(alturaPantalla - 100),
+        reason:
+            'the prototype anchors content to the bottom with two '
+            'flexible spacers (lines 693, 706) so the CTA sits at the '
+            'screen edge — it used to render immediately after the top '
+            'padding instead',
+      );
+    },
+  );
 }