fix: prevent orphan transfers on key fetch and passphrase failures

[Copilot]

Two failure paths in transfer create could leave an incomplete, inaccessible transfer on the server with no cleanup.

Changes

• Sequential key fetch before share creation: GetActiveKey is now called before CreateShare. The previous parallel goroutine pattern meant a share could be created on the server even if the key fetch subsequently failed, with no way to clean it up.

• Disable share on passphrase failure: After share creation, if reading the passphrase from the terminal fails or an empty passphrase is submitted non-interactively, DisableTransfer is now called before returning the error.

// GetActiveKey called first — no share created if this fails
userKey, err := client.GetActiveKey(ctx)
...
share, err := client.CreateShare(ctx, expire, titlePtr, true, toEmails)
...
// Passphrase read failure → clean up the orphan
if err != nil {
    _ = client.DisableTransfer(ctx, share.ID)
    return fmt.Errorf("reading passphrase: %w", err)
}

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