Do not auto generate Display Name (FN) field and allow editing/external updates

[asandikci]

Checklist

• [X] I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• [X] I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• [X] I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• [X] I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• [X] This issue contains only one feature request.
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Feature description

Make "Display Name" (FN) field editable and do not auto update from other fields.

Why do you want this feature?

As far as I can understand contacts app auto generates Display Name (FN) field from First Name/Middle Name/Last Name... sections. But I believe generating only once if contact are creating from this app is enough. Users should be able to directly change this field.

This supports users to easily organize their contacts I guess.

Additional information

Note: This is also required for Full Nextcloud/Vcard v4 Integration. See #85 Note2: Otherwise updating Display Name field from any DAV server (for example Nextcloud + Davx5) are not going to work because app still updates it from other fields.

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[RokeJulianLockhart]

https://github.com/FossifyOrg/Contacts/issues/129#issue-2325712796

@asandikci, if this is implemented, I would like to be able to choose to retain the current behaviour. Were it not doing this, I would have filed an FR to have the FN automatically update from the N field. However, should https://github.com/nextcloud/contacts/issues/3983#issue-2354828434 be implemented, it would be reasonable to expect that this conversation would become necessary.