[Bug]: Reset key mappings

### Pre-flight checklist

- [x] I searched [existing issues](https://github.com/AprilNEA/OpenLogi/issues?q=is%3Aissue) and this is not a duplicate.
- [x] I am on the [latest release](https://github.com/AprilNEA/OpenLogi/releases/latest) or a recent `master` build.
- [x] I quit **Logi Options+** before running OpenLogi (the two apps fight over HID++ access and only one can own a receiver at a time).

### Which part of OpenLogi?

GUI (desktop app)

### OpenLogi version

0.8.3

### Operating system

Windows

### OS version & architecture

11 x64

### Device model

K400+

### How is the device connected?

Logi Bolt receiver

### Affected area(s)

- [ ] Device discovery / detection
- [x] Button remapping
- [ ] DPI control
- [ ] SmartShift
- [ ] Per-application profiles
- [ ] Battery status
- [ ] Settings / configuration (TOML)
- [ ] Auto-update
- [ ] Menu bar / tray
- [ ] Other

### What happened?

I am able change a key mapping let's say from F1 to left click. But there is no option to reset the mapping back to F1.

So I ask you to add an entry at top of the remapping list like "original function". Thanks!

### Steps to reproduce

1. Select key to modify mapping
2. Select an action from the list
3. No way to reset in the GUI, have to edit the config.toml and restart OpenLogi

### Diagnostics report

```markdown

```

### `openlogi list` output

```text

```

### Logs

```shell

```

### macOS permissions (if applicable)

- [ ] OpenLogi has **Accessibility** permission (needed to remap buttons via the event tap).
- [ ] OpenLogi has **Input Monitoring** permission (needed for Bluetooth-direct devices and capture).

Start in the desktop GUI code that builds the button-remapping action list, then trace how mappings are written to config.toml. Add a visible original-function reset choice for the selected key and verify that choosing it restores the device's default mapping without manual configuration edits or a restart.

Written by the indexing model from the issue text.