fix: custom port handling in UI for SFTP

[joshtrichards]

• Resolves: #32773
• Resolves: nextcloud/documentation#941

Summary

Same as #47526 but for SFTP.

TODO

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [x] Tests (unit, integration, api and/or acceptance) are included
• [x] Screenshots before/after for front-end changes
• [x] Documentation (manuals or wiki) has been updated or is not required
• [x] Backports requested where applicable (ex: critical bugfixes)

[joshtrichards]

/backport to stable30

[joshtrichards]

/backport to stable29

[artonge]

Is this the only wiring needed for it to work? :astonished:

[joshtrichards]

Is this the only wiring needed for it to work? 😲

Yes, seems so. Ends up in the correct place in the config when used.