feat: meal tracking, dish recognition UX improvements, English AI prompts

Backend:
- Translate all recognition prompts (receipt, products, dish) from Russian to English
- Add lang parameter to Recognizer interface and pass locale.FromContext in handlers
- DishResult type uses candidates array for multi-candidate responses

Client:
- Add meal tracking: diary provider, date selector, meal type model
- DishResult parser: backward-compatible with legacy flat format and new candidates format
- DishResultScreen: sticky bottom button, full-width portion/meal-type inputs,
  КБЖУ disclaimer moved under nutrition card, add date field to diary POST body
- Recognition prompts now return dish/product names in user's preferred language
- Onboarding, profile, home screen visual updates

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/features/home/home_provider.dart

@@ -3,6 +3,19 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../../shared/models/home_summary.dart';
 import 'home_service.dart';
 
+// ── Selected date (persists while app is open) ────────────────
+
+/// The date currently viewed on the home screen.
+/// Defaults to today; can be changed via the date selector.
+final selectedDateProvider = StateProvider<DateTime>((ref) => DateTime.now());
+
+/// Formats a [DateTime] to the 'YYYY-MM-DD' string expected by the diary API.
+String formatDateForDiary(DateTime date) =>
+    '${date.year}-${date.month.toString().padLeft(2, '0')}-'
+    '${date.day.toString().padLeft(2, '0')}';
+
+// ── Home summary ──────────────────────────────────────────────
+
 class HomeNotifier extends StateNotifier<AsyncValue<HomeSummary>> {
   final HomeService _service;