When auto-select picks an alias identity matching the original recipient,
the alias often has no signature configured. The composer was using the
alias's empty signature for both the visual preview and the appended
signature on send, so neither showed up. New mail worked because no
auto-select runs.
Add a signatureIdentity that falls back to the primary when the current
identity has no signature. From address, identity ID, S/MIME, and draft
saves still use currentIdentity so mail goes out from the right address.
