feat(i18n): add FR/EN internationalization with in-app language selector

- gen-l10n setup (flutter_localizations, intl, l10n.yaml, ARB en/fr)
- localeProvider (persisted) wired into MaterialApp
- language selector in the System page (alongside the color palette)
- all user-facing strings across screens moved to AppLocalizations

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/core/config/app_constants.dart

@@ -41,6 +41,9 @@ class AppConstants {
   /// Clé SharedPreferences pour le filtre de générations.
   static const String prefsGenFilter = 'gen_filter';
 
+  /// Clé SharedPreferences pour la langue de l'application.
+  static const String prefsLocale = 'locale_code';
+
   /// Plages d'IDs Pokémon par génération [min, max] (inclusif).
   static const List<(int, int)> genRanges = [
     (1, 151),    // Gen 1