Improve TXT export readability by using label:value format instead of raw JSON

### Describe your improvement idea or feature request

Currently, exporting tasks as a `.txt` file outputs raw JSON data, which is difficult for users to read and understand.
This enhancement proposes formatting TXT exports in a human-readable `label: value` format while keeping JSON export unchanged.

### How would you like to see it implemented?

1. Exported `.txt` files should display tasks in a readable format such as:
- Description: Buy groceries
- Status: pending
- Due: 2025-01-01 10:00
- Priority: L
- Tags: home, shopping
2. JSON export should remain unchanged and continue to provide raw data.

### Put here any screenshots or videos (optional)

Before:
After:

### How can we contact you (optional)

_No response_

### Would you like to work on this issue?

Yes

### By submitting this issue, I have confirmed that:

- [x] I have starred the repo ⭐ and watched 👀 it on GitHub and followed the contribution guidelines.

Localiza la implementación de exportación de tareas y compara las rutas de exportación TXT y JSON. Actualiza únicamente la salida TXT para que use los campos label: value solicitados; después, verifica que JSON permanezca sin cambios y que las tareas exportadas incluyan los valores legibles indicados.

Escrito por el modelo de indexación a partir del texto del issue.