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/pubspec.yaml

@@ -9,6 +9,9 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
+  flutter_localizations:
+    sdk: flutter
+  intl: ^0.20.1
 
   cupertino_icons: ^1.0.8
 
@@ -52,3 +55,4 @@ dev_dependencies:
 
 flutter:
   uses-material-design: true
+  generate: true