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/features/onboarding/onboarding_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:food_ai/l10n/app_localizations.dart';
 import 'package:go_router/go_router.dart';
 
 import '../../core/theme/app_colors.dart';
@@ -1151,6 +1152,7 @@ class _MealTypesStepContent extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context)!;
     final theme = Theme.of(context);
     return Column(
       crossAxisAlignment: CrossAxisAlignment.start,
@@ -1178,7 +1180,7 @@ class _MealTypesStepContent extends StatelessWidget {
                   const SizedBox(width: 12),
                   Expanded(
                     child: Text(
-                      mealTypeOption.label,
+                      mealTypeLabel(mealTypeOption.id, l10n),
                       style: theme.textTheme.bodyLarge?.copyWith(
                         color: isSelected
                             ? accentColor