Function activate_with 

pub async fn activate_with(
    path: &Path,
    client: &Client,
    key: &str,
    instance_name: &str,
    now: DateTime<Utc>,
) -> Result<SupporterRecord, String>

Pure activation helper: sniffs the key’s source, runs the matching verification path (offline Ed25519 for ENT1-…, Lemon Squeezy API for everything else), and persists the resulting record to disk.

The Tauri command is a thin shim over this so the orchestration can be exercised end-to-end without spinning up a Tauri runtime.