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_confirm_screen.dart

@@ -248,10 +248,14 @@ class _ItemTileState extends State<_ItemTile> {
             const SizedBox(width: 8),
             widget.units.isEmpty
                 ? const SizedBox(width: 48)
-                : DropdownButton<String>(
-                    value: widget.units.containsKey(widget.item.unit)
-                        ? widget.item.unit
-                        : widget.units.keys.first,
+                : Builder(builder: (builderContext) {
+                    // Reconcile item.unit with valid server codes so that the
+                    // submitted value matches what the dropdown displays.
+                    if (!widget.units.containsKey(widget.item.unit)) {
+                      widget.item.unit = widget.units.keys.first;
+                    }
+                    return DropdownButton<String>(
+                    value: widget.item.unit,
                     underline: const SizedBox(),
                     items: widget.units.entries
                         .map((e) => DropdownMenuItem(value: e.key, child: Text(e.value)))
@@ -262,7 +266,8 @@ class _ItemTileState extends State<_ItemTile> {
                         widget.onChanged();
                       }
                     },
-                  ),
+                  );
+                  }),
             IconButton(
               icon: const Icon(Icons.close),
               onPressed: widget.onDelete,