feat: Introduce category selection functionality in NoteEditorScreen and related components

lib/models/note.dart

@@ -1,5 +1,7 @@
 import 'package:uuid/uuid.dart';
 
+const Object _unsetCategoryId = Object();
+
 // Model: Note
 // - Representa una nota guardada en la app.
 class Note {
@@ -36,12 +38,16 @@ class Note {
     DateTime? createdAt,
     DateTime? updatedAt,
     double? position,
-    String? categoryId,
+    Object? categoryId = _unsetCategoryId,
     int? serverVersion,
     bool? isDeleted,
     bool? isPermanentlyDeleted,
     bool? isDirty,
   }) {
+    final String? resolvedCategoryId = identical(categoryId, _unsetCategoryId)
+        ? this.categoryId
+        : categoryId as String?;
+
     return Note(
       id: id ?? this.id,
       title: title ?? this.title,
@@ -49,7 +55,7 @@ class Note {
       createdAt: createdAt ?? this.createdAt,
       updatedAt: updatedAt ?? this.updatedAt,
       position: position ?? this.position,
-      categoryId: categoryId ?? this.categoryId,
+      categoryId: resolvedCategoryId,
       serverVersion: serverVersion ?? this.serverVersion,
       isDeleted: isDeleted ?? this.isDeleted,
       isPermanentlyDeleted: isPermanentlyDeleted ?? this.isPermanentlyDeleted,