feat: dynamic units table with localized names via GET /units

- Add units + unit_translations tables with FK constraints on products and ingredient_mappings
- Normalize products.unit from Russian strings (г, кг) to English codes (g, kg)
- Load units at startup (in-memory registry) and serve via GET /units (language-aware)
- Replace hardcoded _units lists and _mapUnit() functions in Flutter with unitsProvider FutureProvider
- Re-fetches automatically when language changes

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

client/lib/features/menu/shopping_list_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../../core/locale/unit_provider.dart';
 import '../../shared/models/shopping_item.dart';
 import 'menu_provider.dart';
 
@@ -191,14 +192,14 @@ class _ShoppingTile extends ConsumerWidget {
       ),
       subtitle: item.inStock > 0
           ? Text(
-              '${item.inStock.toStringAsFixed(0)} ${item.unit} есть дома',
+              '${item.inStock.toStringAsFixed(0)} ${ref.watch(unitsProvider).valueOrNull?[item.unit] ?? item.unit} есть дома',
               style: theme.textTheme.bodySmall?.copyWith(
                 color: Colors.green,
               ),
             )
           : null,
       trailing: Text(
-        '$amountStr ${item.unit}',
+        '$amountStr ${ref.watch(unitsProvider).valueOrNull?[item.unit] ?? item.unit}',
         style: theme.textTheme.bodyMedium?.copyWith(
           fontWeight: FontWeight.w600,
         ),