[Feature]: Capture Region to storage

### Pre-flight checklist

- [x] I searched existing issues and the [Roadmap](https://github.com/AprilNEA/OpenLogi#roadmap), and this isn't already tracked.

### Problem / motivation

I love to take immediate screenshots using the forward/back buttons on my MX Master. Openlogi options allows the capture of a region and save it to clipboard. Logioptions and MacOS Additionally support to save to a predefined storage location such as Desktop or Downloads.

Both to clipboard as well as to storage are button shortcuts that I find extremely useful especially in business context and it saves me a lot of time.

### Proposed solution

I would like to have either a modifier separate action to use the same capture region feature but with a specific storage destination instead of the clipboard. Non configurable default to Desktop or Downloads would be fine to me.

In my use case I actually have one button mapped to save to clipboard and another to save to Desktop. I need both depending on the situation.

### Alternatives considered

[Feature]: Capture Region to Clipboard

### Related area(s)

- [ ] GUI
- [ ] CLI
- [x] Button actions / remapping
- [ ] DPI
- [ ] SmartShift
- [ ] Per-application profiles
- [ ] Configuration (TOML)
- [ ] Auto-update
- [ ] Other

### Additional context

[Feature]: Capture Region to Clipboard https://github.com/AprilNEA/OpenLogi/issues/288

Start with the button actions/remapping area and the existing Capture Region to Clipboard issue (#288). Trace how the current region-capture action targets the clipboard, then define a separate action that preserves clipboard capture while saving to a predefined Desktop or Downloads location. Done means both shortcuts work independently.

Written by the indexing model from the issue text.