[Feature]: ShortCut

### 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

Everything is great, but there is one problem.
Linux
GUI shortcuts aren't supported.
An even bigger issue is that `config.toml` doesn't support the `Super key`,
yet practically the entire launchpad relies on `Super + key` combinations.

`[devices."receiver:bfdf741af79f6909:slot:6".bindings.GestureButton]
Up·=·{·CustomShortcut·=·"Super+A"·}
`

Thanks. Great job.

### Proposed solution

Add support for the `Super key` in `config.toml`

### Alternatives considered

_No response_

### Related area(s)

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

### Additional context

_No response_

Start by locating the config.toml parsing and the GUI button-binding or shortcut-handling entry points, then trace how CustomShortcut values are interpreted. Done means config.toml accepts the Super key and a configured Super+key shortcut works for GUI button actions on Linux.

Written by the indexing model from the issue text.