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>

backend/internal/domain/user/repository.go

@@ -112,7 +112,7 @@ func buildUpdateQuery(id string, req UpdateProfileRequest) (string, []interface{
 		argIdx++
 	}
 	if req.Preferences != nil {
-		setClauses = append(setClauses, fmt.Sprintf("preferences = $%d", argIdx))
+		setClauses = append(setClauses, fmt.Sprintf("preferences = preferences || $%d::jsonb", argIdx))
 		args = append(args, string(*req.Preferences))
 		argIdx++
 	}