Cannot set arbitrary seconds in add timestamp dialog

### ⚠️ 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 the add timestamp dialog, while you may enter arbitrary values for the hour ('HH') and minute ('mm') components, there is no way to set an arbitrary value for the seconds ('ss') component of the timestamp.

There are two ways to set the time components of the timestamp: by resetting the "current time" variable, or by resetting the "picked time" variable.

When you first open the add timestamp dialog, the "Use the current date & time" toggle is preselected, and all three time components of the "current time" variable are initialized to the time when the dialog is opened. I put the phrase "current time" in quotes, as the "current time" variable stays at that initialized value unless you deselect and then reselect the "Use current date & time" toggle, in which case all three components of the "current time" variable are reinitialized each time you reselect the "Use the current date & time" toggle.

Each time you open the "Pick time" dialog, all three components of the "pick time" variable are reinitialized from the corresponding components of the "current time" variable. (N.B. The "current time" variable itself is not reinitialized by opening the "Pick time" dialog.) The dialog displays a digital or analog clock showing the hour and minute components of the "pick time" variable, which may be changed independently from the "current time" variable. The clock does not display the seconds component of the "pick time" variable, which remains fixed at the value copied from the "current time" variable.

I have attached a short screen recording of the issue.

https://github.com/user-attachments/assets/2bd83655-ce69-47db-99d1-2c96f9769df4

### Steps to reproduce

The steps to reproduce the issue are described fully in the problem description.

### Information

Android version: 14
Build number : U1TFS34.100-35-14-1-2
Device: Motorola moto g play - 2024
App Version: 2.14.1

### Source

Google Play

### Format / File type

Something else (-> Additional info)

### Additional info / Log

```shell
This issue occurred with both plaintext and Markdown, and in both the Google Play release (2.14.1) and Github marder (2.14.1-2012) installations.
```

Start by locating the add timestamp dialog and its "Pick time" and "Use the current date & time" controls in the Android application. Trace how the current time and picked time values are initialized, then verify that the seconds component can be set independently and is retained in the resulting timestamp.

Written by the indexing model from the issue text.