TaskyPad 1.0.6

Modernize CrearTarea UI and add validation logic

Updated the `CrearTarea` window with a modern UI design, including new styles for controls, a scrollable layout, and a fixed alert section for validation errors. Added a `ValidateForm` method to ensure required fields are filled before creating or updating tasks. Introduced a cancel button to close the window.

Enhanced task creation and update logic to include a new `notificar` property, allowing users to enable notifications for tasks. Updated the `Tarea` model and constructor to support this property.

Refactored UI elements, replacing old controls with modern-styled equivalents. Improved user experience by centering the window, disabling resizing, and providing clear validation feedback.

Incremented project version to `1.0.6` and performed minor code cleanup for better readability and maintainability.