Zen/Focus Mode

### ⚠️ 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)_.
- [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

A Zen/Focus mode that goes full screen, hides the UI, and, in landscape mode has wide margins.

### Information

Android version: 15
Device: Pixel 6
App Version: 2.14.1

### Source

F-Droid

### Format / File type

Not specific

### Additional info / Log

```shell

```

The issue does not name any files, tests, or entry points. Start by locating Markor's Android editor and screen UI entry points, then define the fullscreen, hidden-UI, and landscape-margin behavior; done means the mode can be entered and exited and those behaviors work consistently.

Written by the indexing model from the issue text.