feat: Two items in the scheduling section

### Preflight checklist

- [x] I searched [existing issues](https://github.com/ja7ad/hydra/issues?q=is%3Aissue) and this has not been proposed already.
- [x] I checked the [README](https://github.com/ja7ad/hydra/blob/main/README.md) and this is not already supported.

### Component

Desktop GUI (Hydra Download Manager)

### Problem

Hello and best regards.
In the area I’ve marked with an arrow above—specifically within the Download Manager application—there are controls to increase or decrease the value next to the number "4"; however, the Hydra application lacks this feature, requiring the numbers to be entered manually.
Secondly, there is no "Save" function available—meaning that once settings are applied, there is no option to save them—whereas the Download Manager application does include this option.

### Proposed solution

For the first point, I suggest adding the up/down arrows next to the number.
For the second point, I suggest replacing the "Stop" option with a "Save" option so that changes made on this page can be saved.

### Alternatives considered

_No response_

### Platforms this should cover

- [ ] Linux
- [ ] macOS
- [x] Windows
- [ ] Android / iOS (via `libhydra`)

### Additional context

_No response_

### Compatibility

- [ ] This changes existing behavior (flags, defaults, output format, or the C ABI).

### Contribution

- [ ] I'm willing to open a pull request implementing this.

Start in the desktop GUI's scheduling section and trace how the numeric setting and current Stop action are implemented. Compare the existing Download Manager behavior described in the issue, then verify that the scheduling value has visible up/down controls and that settings can be saved on Windows.

Written by the indexing model from the issue text.