[Feature]: Add "Close Window" action

### 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

On macOS with the official Logitech app I've assigned the "Close Window" action to my middle mouse button. This option is missing in OpenLogi.

### Proposed solution

Add a "close window" action to OpenLogi on macOS.

### Alternatives considered

_No response_

### Related area(s)

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

### Additional context

_No response_

The issue names no implementation files or tests. Start by tracing how existing button actions are represented and handled on macOS, then verify the new action can be assigned to a mouse button and closes the focused window.

Written by the indexing model from the issue text.