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/presentation/providers/locale_provider.dart

@@ -0,0 +1,33 @@
+import 'dart:ui';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import '../../core/config/app_constants.dart';
+
+/// Langues prises en charge par l'application.
+const List<Locale> supportedLocales = [Locale('en'), Locale('fr')];
+
+/// Gère la langue sélectionnée, persistée dans les préférences.
+class LocaleNotifier extends Notifier<Locale> {
+  @override
+  Locale build() {
+    _loadSaved();
+    return const Locale('en');
+  }
+
+  Future<void> _loadSaved() async {
+    final prefs = await SharedPreferences.getInstance();
+    final code = prefs.getString(AppConstants.prefsLocale);
+    if (code != null && supportedLocales.any((l) => l.languageCode == code)) {
+      state = Locale(code);
+    }
+  }
+
+  Future<void> setLocale(Locale locale) async {
+    state = locale;
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(AppConstants.prefsLocale, locale.languageCode);
+  }
+}
+
+/// Langue active de l'application.
+final localeProvider = NotifierProvider<LocaleNotifier, Locale>(LocaleNotifier.new);