feat(tutorial): add 9-screen help/tutorial carousel

Add PantallaTutorialAyuda, a PageView-based carousel covering saved
stations/groups, per-station equalizer, recording, adaptive alarms,
Android Auto favorites, auto-reconnect, snooze duration, custom
stations, and a closing summary with a "watch it again" reminder.

ServicioTutorialAyuda persists a one-time seen flag so the carousel
shows once via mostrarSiProcede, independent of entry point; the
final page's CTA label depends on the primerArranque constructor
parameter ("Empezar a escuchar" vs "Cerrar").

Translate the new copy into all 13 supported locales and update
helpSubtitle to describe the new entry point.

lib/l10n/gen/app_localizations_en.dart

@@ -1408,7 +1408,77 @@ class AppLocalizationsEn extends AppLocalizations {
   String get helpTitle => 'Help and tutorial';
 
   @override
-  String get helpSubtitle => 'Review PluriWave features, tips and what’s new.';
+  String get helpSubtitle => '9 screens · watch it again anytime';
+
+  @override
+  String get tutorialSkipAction => 'Skip';
+
+  @override
+  String get tutorialNextAction => 'Next';
+
+  @override
+  String get tutorialPage1Headline => 'Save your stations and group them';
+
+  @override
+  String get tutorialPage1Body =>
+      'Tap the heart to save a station. In \"Your stations\" you can create groups like \"Every morning\" or \"Car\" and drag to reorder them.';
+
+  @override
+  String get tutorialPage2Headline =>
+      'One base equalizer, plus one per station';
+
+  @override
+  String get tutorialPage2Body =>
+      'In Settings you set the general equalizer. And from a specific station\'s playback screen you can give it its own setting, which overrides the general one.';
+
+  @override
+  String get tutorialPage3Headline => 'Record what you\'re listening to';
+
+  @override
+  String get tutorialPage3Body =>
+      'From the player\'s tool tray, \"Record\" saves the original stream. Find your recordings under Settings › Recordings.';
+
+  @override
+  String get tutorialPage4Headline => 'Alarms that adapt to you';
+
+  @override
+  String get tutorialPage4Body =>
+      'Wake up to your favorite station, snooze for 3, 5, or 10 minutes, and add vacation ranges so some alarms skip themselves.';
+
+  @override
+  String get tutorialPage5Headline => 'Your favorites, in the car too';
+
+  @override
+  String get tutorialPage5Body =>
+      'Connect your phone with Android Auto to find Favorites, All stations, Your stations, and your Local Music, with big buttons made for driving.';
+
+  @override
+  String get tutorialPage6Headline => 'It reconnects on its own';
+
+  @override
+  String get tutorialPage6Body =>
+      'If the signal drops, PluriWave retries automatically and keeps showing your saved favorites even without a connection.';
+
+  @override
+  String get tutorialPage7Headline => 'Choose how long to snooze';
+
+  @override
+  String get tutorialPage7Body =>
+      'When an alarm rings, there\'s no single \"snooze\": you choose 3, 5, or 10 minutes depending on what you need at that moment.';
+
+  @override
+  String get tutorialPage8Headline => 'Can\'t find it? Add it yourself';
+
+  @override
+  String get tutorialPage8Body =>
+      'In \"Your stations\" → Add custom station, paste the stream URL of a station that isn\'t in the search results. It\'s saved under \"Your stations\", also available in the car.';
+
+  @override
+  String get tutorialPage9Headline => 'Done — now you know the essentials';
+
+  @override
+  String get tutorialPage9BannerBody =>
+      'To watch this tutorial again anytime: Settings → Information → Help and tutorial.';
 
   @override
   String get indefiniteOption => 'Indefinite';