feat(bienvenida): add monetization-free welcome screen

Build the first-run welcome surface from mockup screen 14, stripped of
its entire monetization block: no PRO pill, no "14 dias PRO gratis"
trial line, no pricing card, no secondary "free version" link. Ships
only logo, headline, body copy, exactly 3 feature bullets, and the
single "Empezar a escuchar" CTA, as a full-screen route (not a modal).

Spanish copy is re-cast from the mockup's "tu" form to the app's
established voseo register (matching ~750 existing app_es.arb lines),
using "auto" instead of "coche" per the one existing precedent. The 3
bullet icons reuse existing tokens (electricMagenta/liveGreen/
warmCoral) that already match the mockup's own hex values for them.

CTA switches to the Escuchar tab and pops the route. Wiring this
screen into the real first-launch flow (main.dart/app.dart) is left
for a follow-up unit, same shape as WU15/WU15b - this WU only covers
the isolated, tested screen per its own task list and verify command.

WU17.

lib/l10n/gen/app_localizations.dart

@@ -2869,6 +2869,60 @@ abstract class AppLocalizations {
   /// In es, this message translates to:
   /// **'Carpeta seleccionada'**
   String get localMusicFolderGenericName;
+
+  /// No description provided for @welcomeHeadline.
+  ///
+  /// In es, this message translates to:
+  /// **'Tu mundo, en directo'**
+  String get welcomeHeadline;
+
+  /// No description provided for @welcomeBody.
+  ///
+  /// In es, this message translates to:
+  /// **'53.412 emisoras de 238 países. Guardá tus favoritas, llevalas al auto y despertate con ellas.'**
+  String get welcomeBody;
+
+  /// No description provided for @welcomeBullet1Title.
+  ///
+  /// In es, this message translates to:
+  /// **'Ecualizador por emisora'**
+  String get welcomeBullet1Title;
+
+  /// No description provided for @welcomeBullet1Subtitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Y un preset por dispositivo de salida'**
+  String get welcomeBullet1Subtitle;
+
+  /// No description provided for @welcomeBullet2Title.
+  ///
+  /// In es, this message translates to:
+  /// **'Android Auto'**
+  String get welcomeBullet2Title;
+
+  /// No description provided for @welcomeBullet2Subtitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Tus favoritas y tu música local en el auto'**
+  String get welcomeBullet2Subtitle;
+
+  /// No description provided for @welcomeBullet3Title.
+  ///
+  /// In es, this message translates to:
+  /// **'Alarmas musicales'**
+  String get welcomeBullet3Title;
+
+  /// No description provided for @welcomeBullet3Subtitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Con subida progresiva y modo vacaciones'**
+  String get welcomeBullet3Subtitle;
+
+  /// No description provided for @welcomeCtaLabel.
+  ///
+  /// In es, this message translates to:
+  /// **'Empezar a escuchar'**
+  String get welcomeCtaLabel;
 }
 
 class _AppLocalizationsDelegate