feat(selection): implement multi-select functionality with action bar (#730) (#566)

test/multi_select_controller_test.dart

@@ -0,0 +1,46 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:proxypin/ui/component/multi_select_controller.dart';
+
+void main() {
+  group('MultiSelectController', () {
+    test('toggle enables and clears selection mode', () {
+      final controller = MultiSelectController();
+
+      controller.toggle('a');
+      expect(controller.isSelectionMode, isTrue);
+      expect(controller.selectedIds, {'a'});
+
+      controller.toggle('a');
+      expect(controller.isSelectionMode, isFalse);
+      expect(controller.selectedIds, isEmpty);
+    });
+
+    test('selectRange uses anchor item', () {
+      final controller = MultiSelectController();
+      controller.selectOnly('b');
+
+      controller.selectRange(['a', 'b', 'c', 'd'], 'd');
+
+      expect(controller.selectedIds, {'b', 'c', 'd'});
+    });
+
+    test('selectAll and prune keep only visible ids', () {
+      final controller = MultiSelectController();
+
+      controller.prune(['b', 'c', 'd']);
+
+      expect(controller.isSelectionMode, isTrue);
+      expect(controller.selectedIds, {'b', 'c'});
+    });
+
+    test('prune clears selection mode when all selected ids disappear', () {
+      final controller = MultiSelectController();
+
+      controller.prune(['c']);
+
+      expect(controller.isSelectionMode, isFalse);
+      expect(controller.selectedIds, isEmpty);
+    });
+  });
+}
+