fix: POST /products 500 — invalid unit FK on unrecognized items

Two bugs caused a FK constraint violation on products.unit:
1. RecognizedItem.fromJson fell back to 'шт' (Cyrillic, not a valid
   units.code) when the AI returned a null unit — changed to 'pcs'.
2. The unit dropdown in RecognitionConfirmScreen displayed units.keys.first
   for invalid units but never updated item.unit, so the invalid value was
   still submitted. Added a reconcile step in build() that syncs item.unit
   to units.keys.first whenever the stored value is not in the valid set.

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

client/lib/features/scan/recognition_service.dart

@@ -39,7 +39,7 @@ class RecognizedItem {
     return RecognizedItem(
       name: json['name'] as String? ?? '',
       quantity: (json['quantity'] as num?)?.toDouble() ?? 1.0,
-      unit: json['unit'] as String? ?? 'шт',
+      unit: json['unit'] as String? ?? 'pcs',
       category: json['category'] as String? ?? 'other',
       confidence: (json['confidence'] as num?)?.toDouble() ?? 0.0,
       mappingId: json['mapping_id'] as String?,