Make hierarchy more visible in TOC

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

- [x] This is a **enhancement/feature request**. Not a [bug or question](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 and checked most recent issues)_.
- [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 wanted feature/enhancement is not 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's settings are not touched. If your desired feature is present, you don't need to open this issue. The change will be part of the next Markor update.)

### Description

Table of Contents displays all titles with the same size (even with the "bigger headings" setting). It's displayed like this, with no markdown rendering:
~~~
# A
## 1
# B
~~~
That may be hard to visually detect which title is above the other. Could we get decreasing title size for subtitles?

### Information

Android version: 16
App Version: 2.16.1

### Source

F-Droid

### Format / File type

Not specific

### Additional info / Log

```shell
-
```

Start by locating the Table of Contents rendering and the setting described as "bigger headings" in the Android application. Compare how heading levels are displayed, then verify that subtitles are visibly smaller than parent headings while the existing TOC content remains intact.

Written by the indexing model from the issue text.