Hot config reload without needing to restart process

### Feature Request

Right now we restart the core (server/client) process to reload the config. This is very inefficient, and results in a clunky UX. It also causes UX problems with things like the libportal permission dialog.

Instead of restarting the core process to apply the config, the GUI should simply write to a config file (e.g. `deskflow-config.toml`) and the core process should re-read the config when the file changes. This can either be triggered by watching the file or the GUI could poke the core process through a file socket. This would also help when running the core process as a service (so the GUI doesn't have control over the core process lifecycle; the OS does).

Related: #7804