Network Proxy Settings

### What

I would like to add the network proxy setting function so that can customize the network proxy.

### Why

Due to the network or Simplenote server is far away from China, when using Simplenote, you will often encounter PC client and mobile client data is not synchronized. By adding proxy settings, users can customize the network proxy to solve the problem of data desynchronization between different clients.

### How

You can refer to Telegram proxy settings as below:


The issue names no files, tests, or entry points. Start by locating the desktop network configuration and synchronization entry points, then determine how proxy settings should be exposed and applied. Done means users can configure a proxy and synchronization works through it.

Written by the indexing model from the issue text.