feat: Introduce category selection functionality in NoteEditorScreen and related components

test/note_test.dart

@@ -0,0 +1,21 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:notas/models/note.dart';
+
+void main() {
+  test('copyWith can clear categoryId explicitly', () {
+    final Note note = Note(
+      title: 'A',
+      body: 'B',
+      createdAt: DateTime(2026, 5, 21),
+      updatedAt: DateTime(2026, 5, 21),
+      position: 0,
+      categoryId: 'cat-1',
+    );
+
+    final Note cleared = note.copyWith(categoryId: null);
+
+    expect(cleared.categoryId, isNull);
+    expect(cleared.id, note.id);
+  });
+}