Function build_sync_status 

pub fn build_sync_status(
    heads: &[DeviceHead],
    our_device_id: &str,
    local_sync_time: Option<&str>,
) -> SyncStatus

Build a SyncStatus from a list of device heads.

our_device_id identifies the local device so its head can be separated from the “other devices” list. local_sync_time is the RFC 3339 timestamp of when we last completed a sync cycle (tracked locally, not from the heads).