feat: Introduce category selection functionality in NoteEditorScreen and related components

test/note_editor_screen_test.dart

@@ -0,0 +1,81 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:notas/models/category.dart';
+import 'package:notas/models/note.dart';
+import 'package:notas/screens/note_editor_screen.dart';
+
+void main() {
+  testWidgets('saves a note when only the category changes', (
+    WidgetTester tester,
+  ) async {
+    Note? savedNote;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: NoteEditorScreen(
+            note: null,
+            categoryId: null,
+            categories: <Category>[
+              Category(
+                id: 'work',
+                name: 'Trabajo',
+                updatedAt: DateTime(2026, 5, 21),
+              ),
+            ],
+            onComplete: (dynamic result) {
+              savedNote = result as Note?;
+            },
+          ),
+        ),
+      ),
+    );
+
+    expect(find.text('Sin categoría'), findsWidgets);
+
+    await tester.tap(find.byKey(const ValueKey<String>('category_selector')));
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.text('Trabajo').last);
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.text('Guardar'));
+    await tester.pumpAndSettle();
+
+    expect(savedNote, isNotNull);
+    expect(savedNote!.categoryId, 'work');
+    expect(savedNote!.title, 'Sin título');
+  });
+
+  testWidgets('only completes once when save is tapped twice', (
+    WidgetTester tester,
+  ) async {
+    int completionCount = 0;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: NoteEditorScreen(
+            note: null,
+            categoryId: null,
+            categories: <Category>[],
+            onComplete: (dynamic result) {
+              if (result is Note) {
+                completionCount += 1;
+              }
+            },
+          ),
+        ),
+      ),
+    );
+
+    await tester.enterText(find.byType(TextField).first, 'Nota de prueba');
+
+    await tester.tap(find.text('Guardar'));
+    await tester.tap(find.text('Guardar'));
+    await tester.pumpAndSettle();
+
+    expect(completionCount, 1);
+  });
+}

test/note_test.dart

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+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);
+  });
+}