Sync / export settings / config

**What problem does this solve?**
I use Harper on multiple devices. I have customized settings (e.g. ignore long sentence linters) that I'd like to apply on multiple devices.

**Proposed Solution**
Either allow some syncing on the cloud or include a button to export / import settings in `json` / `toml` / `yaml` formats.
info.

The issue does not name files, tests, or entry points. First trace how Harper stores customized settings and where settings are exposed across devices, then clarify whether the intended scope is cloud syncing or local import/export. Done requires an agreed approach and a tested way to preserve settings between devices.

Written by the indexing model from the issue text.