To-do widget displays completed tasks incorrectly

### ⚠️ This issue respects the following points: ⚠️

- [X] This is a **bug**. Not a [question or feature request](https://github.com/gsantner/markor/issues/new/choose).
- [X] The topic is **not** already reported at [Issues](https://github.com/gsantner/markor/issues?q=#js-issues-search). _(I've searched it)_.
- [X] Markor **is** up to date. See [Releases](https://github.com/gsantner/markor/tags) for the latest version. Updates are available from [F-Droid](https://f-droid.org/en/packages/net.gsantner.markor/) and GitHub.
- [X] The bug is still present in the latest development version (git master). (Please [download](https://nightly.link/gsantner/markor/workflows/build-android-project/master) and try the test version of Markor, named **Marder**. Don't worry; Markor and Marder appear as completely separate applications. You can install both side-by-side, and Markor settings are not touched. In case the issue is resolved there, you don't need to create a bug report. The change will be part of the next Markor update.)

### Description

The todo.txt widget doesn't seem to handle completed tasks. In my opinion, it should either display them with a strikethrough (like the file view does) or hide them. Instead, it prepends the date.

### Steps to reproduce

1. In the To-Do tab, create a new task, like `2024-12-28 test task`
2. Add the Markor Todo widget (second one) to the home screen. The display of the task will be just "test task" (i.e., start date, priority, etc. are not shown)
3. In markor, use the "Toggle done" button (the checkbox on the far left of the action bar for me). This changes the task to something like `x 2024-12-28 2024-12-28 test task`, and after a moment it renders with a strikethrough. However, the widget instead now shows "2024-12-28 test task" - that is, it grew a date and there's no indication the task has been completed.

### Information

Android version: 14 (LineageOS 21-20241227-NIGHTLY-panther)
Device: Pixel 7
App Version: 2.13.1

### Source

F-Droid

### Format / File type

todo.txt

### Additional info / Log

```shell
-
```

Start with the To-Do widget rendering path and compare it with the file view after using the Toggle done action on a todo.txt task. Reproduce the Android 14 case with a dated task, then verify that completed tasks are either hidden or rendered with a strikethrough rather than showing the date as task content.

Written by the indexing model from the issue text.