Option to keep some notes offline (no sync)

### What
I do not want to sync my notes to the cloud. Allows to have a "on my mac" directory which is not synced across all my devices.

### Why
- For very confidential notes.
- To be sure that my notes are not overwritted due to sync issues
- ...

### How
- A "On my Mac" directory
- Allows to completly disable sync feature

Start with the issue body and map its two requested behaviors: an "On my Mac" directory whose notes never sync, and a way to disable sync completely. Define how each behavior should be verified, then inspect the repository's existing note storage and sync entry points; the issue names no files or tests to begin with.

Written by the indexing model from the issue text.