[Feature]: Add a checkbox to lock the Fn key on the keyboard.

### 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

Add a checkbox to lock the Fn key on the keyboard.

### Proposed solution

Add a checkbox to lock the Fn key on the keyboard. Checking it will lock the key, unchecking it will set it to default.

### Alternatives considered

_No response_

### Related area(s)

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

### Additional context

_No response_

Start by locating the desktop GUI settings and the existing keyboard or Fn-key handling. Determine how the checkbox should connect to the keyboard behavior and how the default state is restored. Done means checking locks Fn, unchecking returns it to default, and the behavior is covered by the project's relevant validation or tests.

Written by the indexing model from the issue text.