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/locale/unit_provider.dart

@@ -0,0 +1,12 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../api/unit_repository.dart';
+import 'language_provider.dart';
+
+/// Fetches and caches units with localized names.
+/// Returns map of code → localized name (e.g. {'g': 'г', 'kg': 'кг'}).
+/// Re-fetches automatically when languageProvider changes.
+final unitsProvider = FutureProvider<Map<String, String>>((ref) {
+  ref.watch(languageProvider); // invalidate when language changes
+  return ref.read(unitRepositoryProvider).fetchUnits();
+});