Allow custom visible From address for JMAP submissions

[cwinters8]

Motivation

• Support contact-form and similar workflows by letting callers set the message's visible From header to the form submitter while choosing an authenticated JMAP identity for the submission.
• Align behavior with JMAP (RFC 8621) where Email headers and EmailSubmission identityId are separate, and servers may enforce permissions on which identity may use which visible From.

Description

• Add SendEmailWithIdentity(from, to, subject, body, identityEmail, isHTML) and make existing SendEmail delegate to it after validating a default From address.
• Add Email.SubmitWithIdentityEmail and Email.SubmitWithIdentityID helpers so EmailSubmission/set can be created with a caller-provided identityId, and factor post-submit mailbox/keyword updates into updateAfterSubmit.
• Keep SubmitCall payload using the provided identityId so EmailSubmission/set follows JMAP requirements, and add validation to Submit for missing From addresses.
• Add unit tests in objects/emails/submit_unit_test.go and document the new API and contact-form usage in README.md.

Testing

• Ran go test ./objects/emails -run 'TestSubmitCallUsesProvidedIdentity|TestSubmitRequiresFromAddressForDefaultIdentity' and the targeted unit tests passed.
• Ran go test ./... which fails in this environment because integration tests require a .env and a valid Fastmail/JMAP session token, causing external-authentication-related failures.

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[cwinters8]

@codex are we ready to merge

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