[Feature]: Global function-key bindings do not work on Linux

### 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

Config docs describe a `[keyboard.bindings]` section for global F-key remapping, e.g.:
```
[keyboard.bindings]
f7 = { OpenApplication = { path = "/usr/bin/gnome-characters", display_name = "Characters" } }
```
On Linux (Fedora 44, GNOME 50, Wayland) this has no effect at all. Pressing the bound key does nothing, not even with the GUI/agent open and correctly reloaded.

### Proposed solution

Implement a Linux backend for global F-key capture.

### Alternatives considered

- Ran agent with `OPENLOGI_LOG=debug` and pressed the bound key: zero log output mentions "keyboard"
- Checked whether the `openlogi-frontmost` GNOME Shell extension was missing

### Related area(s)

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

### Additional context

_No response_

Start by tracing the agent's handling of the [keyboard.bindings] configuration and reproduce the Linux case with OPENLOGI_LOG=debug while pressing F7. Done means the configured global F-key action works on Linux, including when the GUI or agent is running and after configuration reload.

Written by the indexing model from the issue text.