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/scan/recognition_confirm_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 '../products/product_provider.dart';
 import 'recognition_service.dart';
 
@@ -21,8 +22,6 @@ class _RecognitionConfirmScreenState
   late final List<_EditableItem> _items;
   bool _saving = false;
 
-  static const _units = ['г', 'кг', 'мл', 'л', 'шт', 'уп'];
-
   @override
   void initState() {
     super.initState();
@@ -30,7 +29,7 @@ class _RecognitionConfirmScreenState
         .map((item) => _EditableItem(
               name: item.name,
               quantity: item.quantity,
-              unit: _mapUnit(item.unit),
+              unit: item.unit,
               category: item.category,
               mappingId: item.mappingId,
               storageDays: item.storageDays,
@@ -39,27 +38,6 @@ class _RecognitionConfirmScreenState
         .toList();
   }
 
-  String _mapUnit(String unit) {
-    // Backend may return 'pcs', 'g', 'kg', etc. — normalise to display units.
-    switch (unit.toLowerCase()) {
-      case 'g':
-        return 'г';
-      case 'kg':
-        return 'кг';
-      case 'ml':
-        return 'мл';
-      case 'l':
-        return 'л';
-      case 'pcs':
-      case 'шт':
-        return 'шт';
-      case 'уп':
-        return 'уп';
-      default:
-        return unit;
-    }
-  }
-
   @override
   Widget build(BuildContext context) {
     return Scaffold(
@@ -80,7 +58,7 @@ class _RecognitionConfirmScreenState
               itemCount: _items.length,
               itemBuilder: (_, i) => _ItemTile(
                 item: _items[i],
-                units: _units,
+                units: ref.watch(unitsProvider).valueOrNull ?? {},
                 onDelete: () => setState(() => _items.removeAt(i)),
                 onChanged: () => setState(() {}),
               ),
@@ -173,7 +151,7 @@ class _ItemTile extends StatefulWidget {
   });
 
   final _EditableItem item;
-  final List<String> units;
+  final Map<String, String> units;
   final VoidCallback onDelete;
   final VoidCallback onChanged;
 
@@ -268,21 +246,23 @@ class _ItemTileState extends State<_ItemTile> {
               ),
             ),
             const SizedBox(width: 8),
-            DropdownButton<String>(
-              value: widget.units.contains(widget.item.unit)
-                  ? widget.item.unit
-                  : widget.units.last,
-              underline: const SizedBox(),
-              items: widget.units
-                  .map((u) => DropdownMenuItem(value: u, child: Text(u)))
-                  .toList(),
-              onChanged: (v) {
-                if (v != null) {
-                  setState(() => widget.item.unit = v);
-                  widget.onChanged();
-                }
-              },
-            ),
+            widget.units.isEmpty
+                ? const SizedBox(width: 48)
+                : DropdownButton<String>(
+                    value: widget.units.containsKey(widget.item.unit)
+                        ? widget.item.unit
+                        : widget.units.keys.first,
+                    underline: const SizedBox(),
+                    items: widget.units.entries
+                        .map((e) => DropdownMenuItem(value: e.key, child: Text(e.value)))
+                        .toList(),
+                    onChanged: (v) {
+                      if (v != null) {
+                        setState(() => widget.item.unit = v);
+                        widget.onChanged();
+                      }
+                    },
+                  ),
             IconButton(
               icon: const Icon(Icons.close),
               onPressed: widget.onDelete,