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/core/api/language_repository.dart

@@ -8,8 +8,8 @@ class LanguageRepository {
   LanguageRepository(this._api);
 
   Future<Map<String, String>> fetchLanguages() async {
-    final response = await _api.dio.get('/languages');
-    final List<dynamic> items = response.data['languages'] as List;
+    final data = await _api.get('/languages');
+    final List<dynamic> items = data['languages'] as List;
     return {
       for (final item in items)
         item['code'] as String: item['native_name'] as String,

client/lib/core/api/unit_repository.dart

@@ -0,0 +1,22 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../auth/auth_provider.dart';
+import 'api_client.dart';
+
+class UnitRepository {
+  final ApiClient _api;
+  UnitRepository(this._api);
+
+  Future<Map<String, String>> fetchUnits() async {
+    final data = await _api.get('/units');
+    final List<dynamic> items = data['units'] as List;
+    return {
+      for (final item in items)
+        item['code'] as String: item['name'] as String,
+    };
+  }
+}
+
+final unitRepositoryProvider = Provider<UnitRepository>(
+  (ref) => UnitRepository(ref.watch(apiClientProvider)),
+);