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>

CLAUDE.md

@@ -91,3 +91,53 @@ Single- or two-letter names are only allowed when the abbreviation *is* the full
 This rule applies to all languages in this repo (Go and Dart).
 In Go, name variables to avoid shadowing the `error` built-in and the `context` package —
 use descriptive prefixes: `parseError`, `requestContext`, etc.
+
+## Flutter Client Localisation
+
+**Rule:** Every UI string in `client/` must go through `AppLocalizations`.
+Never hardcode user-visible text in Dart source files.
+
+### Current setup
+
+- `flutter_localizations` (sdk: flutter) + `intl: ^0.20.2` in `client/pubspec.yaml`
+- `generate: true` under `flutter:` in `client/pubspec.yaml`
+- `client/l10n.yaml` — generator config (arb-dir, template-arb-file, output-class)
+- Generated class: `package:food_ai/l10n/app_localizations.dart`
+
+### Supported languages
+
+`en`, `ru`, `es`, `de`, `fr`, `it`, `pt`, `zh`, `ja`, `ko`, `ar`, `hi`
+
+### ARB files
+
+All translation files live in `client/lib/l10n/`:
+
+- `app_en.arb` — English (template / canonical)
+- `app_ru.arb`, `app_es.arb`, `app_de.arb`, `app_fr.arb`, `app_it.arb`
+- `app_pt.arb`, `app_zh.arb`, `app_ja.arb`, `app_ko.arb`, `app_ar.arb`, `app_hi.arb`
+
+### Usage pattern
+
+```dart
+import 'package:food_ai/l10n/app_localizations.dart';
+
+// Inside build():
+final l10n = AppLocalizations.of(context)!;
+Text(l10n.someKey)
+```
+
+### Adding new strings
+
+1. Add the key + English value to `client/lib/l10n/app_en.arb` (template file).
+2. Add the same key with the correct translation to **all other 11 ARB files**.
+3. Run `flutter gen-l10n` inside `client/` to regenerate `AppLocalizations`.
+4. Use `l10n.<newKey>` in the widget.
+
+For parameterised strings use the ICU placeholder syntax in ARB files:
+
+```json
+"queuePosition": "Position {position}",
+"@queuePosition": { "placeholders": { "position": { "type": "int" } } }
+```
+
+Then call `l10n.queuePosition(n)` in Dart.