action_share_html_source

### ⚠️ 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

When selecting “Share → HTML source”, the app incorrectly shares rendered HTML content (via convertMarkup()) while declaring the MIME type text/plain. This causes recipients to see HTML tags instead of the expected raw markup (e.g., Markdown, Org, AsciiDoc).

### Steps to reproduce

1. Open any non-HTML document (e.g., Markdown) with headings or formatting symbols.

2. Tap Share → HTML source.

3. Share to a plain-text app (e.g., Notes, Email, SMS).

### Information

Android version:
Device: Samsung S9
App Version:

### Source

F-Droid

### Format / File type

Not specific

### Additional info / Log

```shell
-
```

Start at the Share → HTML source action and trace the convertMarkup() path that prepares the shared content. Reproduce it with a formatted non-HTML document and verify that the action shares the original markup while declaring text/plain, rather than rendered HTML.

Written by the indexing model from the issue text.