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/main.dart

@@ -1,9 +1,12 @@
+import 'dart:ui' show PlatformDispatcher;
+
 import 'package:firebase_core/firebase_core.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import 'app.dart';
+import 'core/locale/language_provider.dart';
 import 'core/storage/local_preferences.dart';
 import 'core/storage/local_preferences_provider.dart';
 import 'firebase_options.dart';
@@ -16,12 +19,22 @@ void main() async {
 
   final sharedPreferences = await SharedPreferences.getInstance();
 
+  // Detect system language on first launch.
+  const supportedLanguageCodes = {
+    'en', 'ru', 'es', 'de', 'fr', 'it', 'pt', 'zh', 'ja', 'ko', 'ar', 'hi',
+  };
+  final systemLanguageCode = PlatformDispatcher.instance.locale.languageCode;
+  final initialLanguage = supportedLanguageCodes.contains(systemLanguageCode)
+      ? systemLanguageCode
+      : 'en';
+
   runApp(
     ProviderScope(
       overrides: [
         localPreferencesProvider.overrideWithValue(
           LocalPreferences(sharedPreferences),
         ),
+        languageProvider.overrideWith((ref) => initialLanguage),
       ],
       child: const App(),
     ),