TOC Links Aren't Functioning

### ⚠️ 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

In my note-taking, the level-4 heading links aren't functioning properly - and those are the only heading levels I normally use.

### Steps to reproduce

1. Remove extra blank lines before H4 headings.
2. Append a placeholder line to the document end.

### Information

Android version: Android14
Device: redminote13
App Version: 2.14.0

### Source

GitHub Releases

### Format / File type

Markdown

### Additional info / Log

```shell
-
```

Reproduce the issue in Markor 2.14.0 or the latest development version using a Markdown note with level-4 headings, no extra blank lines before them, and a placeholder line at the end. Trace the table-of-contents link handling from that reproduction; done means links to level-4 headings navigate correctly under the reported document structure.

Written by the indexing model from the issue text.