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/core/router/app_router.dart

@@ -2,6 +2,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
+import '../../l10n/app_localizations.dart';
+
 import '../auth/auth_provider.dart';
 import '../../features/auth/login_screen.dart';
 import '../../features/auth/register_screen.dart';
@@ -208,15 +210,17 @@ class MainShell extends ConsumerWidget {
           orElse: () => 0,
         );
 
+    final l10n = AppLocalizations.of(context)!;
+
     return Scaffold(
       body: child,
       bottomNavigationBar: BottomNavigationBar(
         currentIndex: currentIndex,
         onTap: (index) => context.go(_tabs[index]),
         items: [
-          const BottomNavigationBarItem(
-            icon: Icon(Icons.home),
-            label: 'Главная',
+          BottomNavigationBarItem(
+            icon: const Icon(Icons.home),
+            label: l10n.navHome,
           ),
           BottomNavigationBarItem(
             icon: Badge(
@@ -224,19 +228,19 @@ class MainShell extends ConsumerWidget {
               label: Text('$expiringCount'),
               child: const Icon(Icons.kitchen),
             ),
-            label: 'Продукты',
+            label: l10n.navProducts,
           ),
-          const BottomNavigationBarItem(
-            icon: Icon(Icons.calendar_month),
-            label: 'Меню',
+          BottomNavigationBarItem(
+            icon: const Icon(Icons.calendar_month),
+            label: l10n.menu,
           ),
-          const BottomNavigationBarItem(
-            icon: Icon(Icons.menu_book),
-            label: 'Рецепты',
+          BottomNavigationBarItem(
+            icon: const Icon(Icons.menu_book),
+            label: l10n.navRecipes,
           ),
-          const BottomNavigationBarItem(
-            icon: Icon(Icons.person),
-            label: 'Профиль',
+          BottomNavigationBarItem(
+            icon: const Icon(Icons.person),
+            label: l10n.profileTitle,
           ),
         ],
       ),