Allow separate signatures for delegated accounts

Is your feature request related to a problem? Please describe.

When using a delegated account, changing the signature also changes the signature in the original account settings. This makes it hard for delegatees to use their own signature when sending on behalf of someone else.

Describe the solution you'd like

A delegated account should be able to have its own signature, separate from the delegator's original account signature, so users can maintain a distinct signature when sending on behalf of someone else.

Describe alternatives you've considered

No clean workaround was identified in the original delegation tracking issue. Manually changing the shared signature is not a good workaround because it affects the original account settings.

Additional context

Reported in https://github.com/nextcloud/mail/issues/12403#issuecomment-4646626237 and split out from the account delegation tracking issue.

Start by tracing how delegated accounts read and save signatures, using the delegation tracking issue and the reported behavior as entry points. Determine where the delegator's signature is shared with the delegated account, then define separate storage and sending behavior. Done means a delegatee can maintain a distinct signature without changing the original account's signature.