Fix cloud sync when rclone progress is enabled

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Summary

• force terminal progress off for sync commands so the JSON log parser receives only JSON records
• append the override after custom Rclone arguments; the command-line value also overrides RCLONE_PROGRESS
• add a regression test for the effective argument order and required JSON logging flags

Testing

• git diff --check
• Rust tests were not run locally because this machine does not have a Rust toolchain; upstream CI will compile and test the change

Fixes #627

[mtkennerly]

Thanks! Tested and this works great.