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).

lib/pantallas/pantalla_bienvenida.dart

@@ -1,3 +1,5 @@
+import 'dart:ui';
+
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
@@ -47,14 +49,31 @@ class PantallaBienvenida extends StatelessWidget {
     final theme = Theme.of(context);
 
     return Scaffold(
-      body: SafeArea(
+      body: Stack(
+        fit: StackFit.expand,
+        children: [
+          const _FondoBienvenida(),
+          SafeArea(
+            child: Padding(
+              padding: const EdgeInsets.fromLTRB(24, 0, 24, 28),
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.stretch,
+                children: [
+                  // Audit 14.2 (t4 lines 693/706): the prototype anchors
+                  // this content to the BOTTOM with two flexible spacers,
+                  // so the CTA sits at the screen edge. The scrollable
+                  // Expanded region above the CTA keeps this safe on short
+                  // screens / large text scale instead of a rigid Spacer
+                  // that could overflow.
+                  Expanded(
                     child: SingleChildScrollView(
-          padding: const EdgeInsets.fromLTRB(24, 48, 24, 28),
+                      padding: const EdgeInsets.only(top: 48),
                       child: Column(
                         crossAxisAlignment: CrossAxisAlignment.stretch,
                         children: [
                           Image.asset(
                             'assets/icons/pluriwave_app_mark.png',
+                            key: const ValueKey('welcome-logo'),
                             width: 76,
                             height: 76,
                             errorBuilder:
@@ -77,7 +96,9 @@ class PantallaBienvenida extends StatelessWidget {
                           Text(
                             l10n.welcomeBody,
                             style: theme.textTheme.bodyMedium?.copyWith(
-                  color: theme.colorScheme.onSurface.withValues(alpha: 0.72),
+                              color: theme.colorScheme.onSurface.withValues(
+                                alpha: 0.72,
+                              ),
                               height: 1.5,
                             ),
                           ),
@@ -102,7 +123,11 @@ class PantallaBienvenida extends StatelessWidget {
                             titulo: l10n.welcomeBullet3Title,
                             subtitulo: l10n.welcomeBullet3Subtitle,
                           ),
-              const SizedBox(height: 32),
+                        ],
+                      ),
+                    ),
+                  ),
+                  const SizedBox(height: 24),
                   SizedBox(
                     height: 58,
                     child: FilledButton(
@@ -120,6 +145,8 @@ class PantallaBienvenida extends StatelessWidget {
               ),
             ),
           ),
+        ],
+      ),
     );
   }
 
@@ -186,3 +213,52 @@ class FilaCaracteristicaBienvenida extends StatelessWidget {
     );
   }
 }
+
+/// Full-bleed blurred backdrop (audit 14.1, t4 lines 687-689). Reuses the
+/// bundled `aurora_wave_banner.png` — the same asset the now-deleted
+/// `PluriScreenHeader` used (Tier 1, S2) — rather than the prototype's own
+/// mockup-only `assets/banner.jpg`, which this app does not bundle. Mirrors
+/// `_FondoArteDifuminado` in `pantalla_alarma_sonando.dart`: blur + a
+/// deepViolet gradient, no noise layer, purely decorative and not
+/// spec-tested beyond its own presence.
+class _FondoBienvenida extends StatelessWidget {
+  const _FondoBienvenida();
+
+  @override
+  Widget build(BuildContext context) {
+    final tokens = context.pluriTokens;
+    return Positioned.fill(
+      key: const ValueKey('welcome-background-art'),
+      child: Stack(
+        fit: StackFit.expand,
+        children: [
+          ImageFiltered(
+            imageFilter: ImageFilter.blur(sigmaX: 20, sigmaY: 20),
+            child: Opacity(
+              opacity: 0.4,
+              child: Image.asset(
+                'assets/images/aurora_wave_banner.png',
+                fit: BoxFit.cover,
+                errorBuilder: (_, __, ___) => const SizedBox.shrink(),
+              ),
+            ),
+          ),
+          DecoratedBox(
+            decoration: BoxDecoration(
+              gradient: LinearGradient(
+                begin: Alignment.topCenter,
+                end: Alignment.bottomCenter,
+                colors: [
+                  tokens.deepViolet.withValues(alpha: 0.45),
+                  tokens.deepViolet.withValues(alpha: 0.9),
+                  tokens.deepViolet,
+                ],
+                stops: const [0, 0.46, 1],
+              ),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}

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(
+      RegExp(r'\d+\s*d[ií]as?\b', caseSensitive: false).hasMatch(textoCompleto),
-        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()
+      final navegacion = EstadoNavegacionRaiz()..irA(RaizPluriWave.ajustes);
-        ..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',
+      );
+    },
+  );
 }