feat: out of office forward to 2

[ixgate-lpeters]

Summary

Adds the ability to forward incoming mails to another address (e.g. a substitute/colleague) while out of office is enabled, in addition to the existing vacation auto-reply.

Previously, the out-of-office feature only supported sending an automated reply to incoming mails. This PR extends the feature so users can optionally specify a forwardTo address, which is added as a Sieve redirect action alongside the vacation response.

Changes

• Add optional forwardTo parameter to OutOfOfficeController::setOutOfOfficeState() (or relevant method)
• Validate forwardTo as a proper email address, throwing a ClientException on invalid input
• Extend OutOfOfficeState and OutOfOfficeParser to support the new forwardTo field
• Extend Sieve script generation to include a redirect action when a forwarding address is configured
• Add forwardTo input field to OutOfOfficeForm.vue
• Add/extend unit tests:
  • OutOfOfficeParserTest
  • OutOfOfficeStateTest
  • New Sieve fixture: sieve-vacation-on-forward-to.sieve

Screenshots

How to test

1. Go to Mail Settings → Out of Office
2. Enable out of office and fill in start date, subject, and message
3. Enter a valid email address in the new "Forward to" field
4. Save the settings
5. Verify:
   • The generated Sieve script contains a redirect action to the specified address
   • Sending a test mail to the account triggers both the vacation auto-reply and a forward to the specified address
6. Test validation:
   • Enter an invalid email in "Forward to" → should show a client-side/server-side validation error
   • Leave "Forward to" empty → out-of-office should work as before (auto-reply only, no forwarding)

TODO / Open questions

• Update user documentation / changelog

Checklist

• Unit tests added/updated
• Manual testing done
• Documentation updated (if needed)
• Changelog entry added (if required by project convention)

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