feat: Refactor inventory screens with category selection and product handling improvements

flutter/lib/core/utils/display_labels.dart

@@ -0,0 +1,9 @@
+String? normalizedOptionalText(String? value) {
+  final normalized = value?.trim();
+  if (normalized == null || normalized.isEmpty) return null;
+  return normalized;
+}
+
+String l1CategoryChipLabel(String prefix, String l1Category) => '$prefix$l1Category';
+
+String locationLabel(String prefix, String location) => '$prefix$location';