feat: Flutter client localisation (12 languages)

Add flutter_localizations + intl, 12 ARB files (en/ru/es/de/fr/it/pt/zh/ja/ko/ar/hi),
replace all hardcoded Russian UI strings with AppLocalizations, detect system locale
on first launch, localise bottom nav bar labels, document rule in CLAUDE.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/app.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:food_ai/l10n/app_localizations.dart';
 
+import 'core/locale/language_provider.dart';
 import 'core/router/app_router.dart';
 import 'core/theme/app_theme.dart';
 
@@ -10,12 +12,16 @@ class App extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final router = ref.watch(routerProvider);
+    final languageCode = ref.watch(languageProvider);
 
     return MaterialApp.router(
       title: 'FoodAI',
       theme: appTheme(),
       routerConfig: router,
       debugShowCheckedModeBanner: false,
+      locale: Locale(languageCode),
+      localizationsDelegates: AppLocalizations.localizationsDelegates,
+      supportedLocales: AppLocalizations.supportedLocales,
     );
   }
 }