Add CLI flag to configure checkpoint file location

[Deletus1]

The sync agent currently always creates its checkpoint file (checkpoint.json) in the current working directory.
This makes it difficult to:

• Keep the development environment clean (checkpoint.json is created in the source directory during builds)
• Align with custom filesystem layouts used in real deployments (e.g. production Electron apps)

In a project I was working on, I needed to control where the checkpoint file is stored.

Solution

This PR introduces a configurable checkpoint file location that can be set via a CLI flag.

Changes

• Added a new checkpoint setting to configuration (settings.Settings.CheckpointFile)
• Added a CLI flag to specify a custom checkpoint file path

Usage

aw-sync-agent -checkpoint=/path/to/checkpoint.json

Notes

• Default behavior is unchanged: checkpoint.json is created in the current working directory when the new CLI flag is not used.