feat: add an owner + sell screen

lib/ui/sell_page/view_model/sell_view_model.dart

@@ -0,0 +1,39 @@
+import 'package:flutter/widgets.dart';
+import 'package:mobile_scanner/mobile_scanner.dart';
+import 'package:seshat/domain/models/book_instance.dart';
+
+class SellViewModel extends ChangeNotifier {
+  SellViewModel();
+
+  bool _showScan = false;
+  bool get showScan => _showScan;
+  set showScan(bool newValue) {
+    _showScan = newValue;
+    notifyListeners();
+  }
+
+  final List<BookInstance> _scannedBooks = [];
+  get scannedBooks => _scannedBooks;
+  void scanBook(BarcodeCapture barcode) {
+    final addedBook = BookInstance(
+      balId: 5,
+      bookId: 5,
+      id: _scannedBooks.length,
+      ownerId: 5,
+      price: 5,
+      status: true,
+    );
+    _scannedBooks.add(addedBook);
+    notifyListeners();
+  }
+
+  void sendSell() {
+    _scannedBooks.clear();
+    notifyListeners();
+  }
+
+  void deleteBook(int id) {
+    _scannedBooks.removeWhere((book) => book.id == id);
+    notifyListeners();
+  }
+}