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/widgets/system/language_picker.dart

@@ -0,0 +1,60 @@
+import 'package:flutter/material.dart';
+import '../../providers/locale_provider.dart';
+
+/// Sélecteur de langue de l'application (parmi [supportedLocales]).
+class LanguagePicker extends StatelessWidget {
+  final Locale selected;
+  final Color primaryColor;
+  final void Function(Locale) onSelect;
+
+  const LanguagePicker({
+    super.key,
+    required this.selected,
+    required this.primaryColor,
+    required this.onSelect,
+  });
+
+  static const _names = {'en': 'English', 'fr': 'Français'};
+  static const _flags = {'en': '🇬🇧', 'fr': '🇫🇷'};
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      children: supportedLocales.map((locale) {
+        final isSelected = locale.languageCode == selected.languageCode;
+        return GestureDetector(
+          onTap: () => onSelect(locale),
+          child: AnimatedContainer(
+            duration: const Duration(milliseconds: 200),
+            margin: const EdgeInsets.only(bottom: 10),
+            padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 14),
+            decoration: BoxDecoration(
+              color: isSelected ? primaryColor : Colors.white,
+              borderRadius: BorderRadius.circular(12),
+              border: Border.all(
+                color: isSelected ? primaryColor : Colors.grey.shade300,
+                width: isSelected ? 3 : 1.5,
+              ),
+            ),
+            child: Row(
+              children: [
+                Text(_flags[locale.languageCode] ?? '', style: const TextStyle(fontSize: 22)),
+                const SizedBox(width: 16),
+                Text(
+                  _names[locale.languageCode] ?? locale.languageCode,
+                  style: TextStyle(
+                    fontSize: 16,
+                    fontWeight: FontWeight.w600,
+                    color: isSelected ? Colors.white : Colors.black87,
+                  ),
+                ),
+                const Spacer(),
+                if (isSelected) const Icon(Icons.check_circle, color: Colors.white, size: 22),
+              ],
+            ),
+          ),
+        );
+      }).toList(),
+    );
+  }
+}

lib/presentation/widgets/system/system_header.dart

@@ -3,7 +3,8 @@ import 'package:flutter/material.dart';
 /// En-tête de la page Système.
 class SystemHeader extends StatelessWidget {
   final Color primaryColor;
-  const SystemHeader({super.key, required this.primaryColor});
+  final String title;
+  const SystemHeader({super.key, required this.primaryColor, required this.title});
 
   @override
   Widget build(BuildContext context) {
@@ -14,10 +15,10 @@ class SystemHeader extends StatelessWidget {
         color: primaryColor,
         border: Border(bottom: BorderSide(color: primaryColor.withAlpha(180), width: 3)),
       ),
-      child: const Text(
-        'SYSTÈME',
+      child: Text(
+        title,
         textAlign: TextAlign.center,
-        style: TextStyle(color: Colors.white, fontSize: 28, fontWeight: FontWeight.bold, letterSpacing: 4),
+        style: const TextStyle(color: Colors.white, fontSize: 28, fontWeight: FontWeight.bold, letterSpacing: 4),
       ),
     );
   }