Android back button exits editor without saving changes in Nextcloud Text

### ⚠️ Before posting ⚠️

- [x] This is a **bug**, not a question or an enhancement.
- [x] I've [searched for similar issues](https://github.com/nextcloud/android/issues) and didn't find a duplicate.
- [x] I've written a clear and descriptive title for this issue, not just "Bug" or "Crash".
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/).

### Steps to reproduce

1. Open a Markdown or text document in Nextcloud Text on an Android device.
2. Edit the document.
3. Press the **Android system back button** to leave the editor.

### Expected behaviour

Pressing the Android back button should either:

* Trigger the same save-and-close behavior as the editor **“X” close button**, or
* Show a warning dialog indicating that there are unsaved changes.

Ideally, the back button should behave consistently with the editor UI close action.

### Actual behaviour

* The editor closes immediately when pressing back button.
* Recent edits are not always saved and file mtime doesn't change ever.
* No warning about unsaved changes is shown.
* The document reopens sometimes with the correct content and other times with the previous content.
* Using the **“X” close button** correctly saves the document.

### Android version

16

### Device brand and model

Samsung S24 Exynos

### Stock or custom OS?

Stock

### Nextcloud android app version

33.0.1

### Nextcloud server version

32.0.16

### Using a reverse proxy?

No

### Android logs

_No response_

### Server error logs

```bash

```

### Additional information

_No response_