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_en.dart

@@ -1581,4 +1581,34 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get localMusicFolderGenericName => 'Selected folder';
+
+  @override
+  String get welcomeHeadline => 'Your world, live';
+
+  @override
+  String get welcomeBody =>
+      '53,412 stations from 238 countries. Save your favorites, take them to the car, and wake up to them.';
+
+  @override
+  String get welcomeBullet1Title => 'Per-station equalizer';
+
+  @override
+  String get welcomeBullet1Subtitle => 'Plus a preset per output device';
+
+  @override
+  String get welcomeBullet2Title => 'Android Auto';
+
+  @override
+  String get welcomeBullet2Subtitle =>
+      'Your favorites and local music in the car';
+
+  @override
+  String get welcomeBullet3Title => 'Music alarms';
+
+  @override
+  String get welcomeBullet3Subtitle =>
+      'With gradual volume rise and vacation mode';
+
+  @override
+  String get welcomeCtaLabel => 'Start listening';
 }