feat: add temp storage shelf (B prefix) UI support

Recognize B-prefix location codes as temp storage shelves. Add purple
color scheme, conflict detection for temp_stored status, and updated
submit/dialog logic for temp storage target handling.

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

lib/pages/registration/parts/registration_overview_part.dart

@@ -19,6 +19,14 @@ extension _RegistrationOverviewPart on _RegistrationPageState {
     );
   }
 
+  /// Find all scanned items that are in temp storage.
+  List<PaichaOverviewItem> _findTempStoredItemsInScanned() {
+    return registration_calculations.findTempStoredItemsInScanned(
+      overview: _overview,
+      zongpaiNos: _zongpaiNos,
+    );
+  }
+
   Future<String?> _baseUrl() async {
     final configService = AppConfigService();
     final baseUrl = await configService.getString('api_url') ?? '';