Some issues affecting visually impaired users

### ⚠️ 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

As a visually impaired user, I frequently use this application because it performs well in terms of accessibility. However, as a blind user, there are currently several serious issues that significantly impact usability.

**File Preview**
When previewing files while editing, such as markdown files, various problems often occur in the preview view:
- The preview page is completely unresponsive to touch.
- The page is touchable, but certain elements—such as lists and images—cannot be read.
- Repeatedly re-entering the preview page may resolve some issues, but new problems often arise.

**Accessibility Labels for Buttons**
Some buttons lack accessibility descriptions, such as those in the search dialog and the buttons next to the text editor.

**Document Reading**
When a document exceeds a certain length, the screen reader fails to continue reading the text. The text editor becomes completely inaccessible to touch, making editing impossible. This issue severely affects usability—once a document reaches a certain length, it becomes unusable.

These are the most pressing issues I have encountered. I hope they can be improved.

### Steps to reproduce

I searched for relevant issues and discussions. Records were found that may affect the issue.
[thes](https://github.com/gsantner/markor/pull/1130)

### Information

Android version: 15
Device: Redmi K70
App Version: 2.14.1

### Source

F-Droid

### Format / File type

Something else (-> Additional info)

### Additional info / Log

```shell
nothing-
```

Start by reviewing the related pull request 1130 and reproducing the reported behavior on Android 15 with a screen reader. Check the file preview, search dialog, editor-side buttons, and long-document reading paths; done means the affected accessibility and interaction failures are isolated and verified as resolved.

Written by the indexing model from the issue text.