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/app_es.arb

@@ -758,5 +758,14 @@
       "error": {}
     }
   },
-  "localMusicFolderGenericName": "Carpeta seleccionada"
+  "localMusicFolderGenericName": "Carpeta seleccionada",
+  "welcomeHeadline": "Tu mundo, en directo",
+  "welcomeBody": "53.412 emisoras de 238 países. Guardá tus favoritas, llevalas al auto y despertate con ellas.",
+  "welcomeBullet1Title": "Ecualizador por emisora",
+  "welcomeBullet1Subtitle": "Y un preset por dispositivo de salida",
+  "welcomeBullet2Title": "Android Auto",
+  "welcomeBullet2Subtitle": "Tus favoritas y tu música local en el auto",
+  "welcomeBullet3Title": "Alarmas musicales",
+  "welcomeBullet3Subtitle": "Con subida progresiva y modo vacaciones",
+  "welcomeCtaLabel": "Empezar a escuchar"
 }