Zim's view mode shows header if timezone offset is negative

Nobody has claimed this yet.

# IMPORTANT
* Package: net.gsantner.markor
* Version: v2.14.1 (158)
* Flavor: Gplay (release)
* Build date: 2025-03-11T01:26Z
* ISource: Google Play
* VCS Hash: 7dee1b72a7cde6f34e5d48b3ca8932652cc8bd85
* VCS Msg: Markor v2.14.1
* Device: Samsung Galaxy S24
* I accept that this issue will be closed without further notice in case I didn't describe the problem including general information, or didn't [search](https://github.com/gsantner/markor/issues?q=) for similar issues & [requests](https://github.com/gsantner/markor/discussions?discussions_q=).

## Explain the Problem
I found a small quirk in the view mode for Zim's format. I noticed that some pages were showing the header of the file, while others didn't. I realized this was due to the timezone offset in the `Creation-Date` timestamp.

When the offset is positive, the view mode works as expected (the header information is hidden):


But when the timezone offset is negative, the header appears above the title:


## Steps to Reproduce

1. Create a page with a positive timezone offset
2. Create a page with a negative timezone offset (using hyphen or minus sign)
3. Open them in view mode

**Page +offset:**
```
Content-Type: text/x-zim-wiki
Wiki-Format: zim 0.6
Creation-Date: 2025-04-19T12:47:15+02:00

====== Zim's view mode with positive timezone offset ======
Created Saturday 19 April 2025

This page looks fine. The timestamp is:

2025-04-19T12:47:15+02:00
```

**Page -offset:**
```
Content-Type: text/x-zim-wiki
Wiki-Format: zim 0.6
Creation-Date: 2025-04-19T12:47:46-02:00

====== Zim's view mode with negative timezone offset ======
Created Saturday 19 April 2025

This page shows the header. The timestamp is:

2025-04-19T12:47:46-02:00
```

Reproduce the issue in Markor's Zim view mode using the positive and negative timezone examples in the report. Trace how the Creation-Date header is detected and hidden, then verify that pages with negative offsets hide the header just like pages with positive offsets.

Written by the indexing model from the issue text.