Optionally fallback to device passkeys/offer selection for where passkeys should be stored/retrieved

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

The KeepassXC-browser extension has a mode to fallback on the system/browser passkeys for when no passkey is fond in the KeePassXC database.
KeePassDX does not have this, if it is not found it may not be possible to enable it.

The following use cases come to mind:
* I may deliberately choose to safe a PassKey not in KeePassDX, but on the local device only. (Notably many web services will want you to label the PassKey when registering, which should show which passkey it is/where it is saved, so that one can revoke it/access when it is no longer valid? So in any case, this decision has then already be made in my mind, and I was presented with the "wrong" location for the passkey)
* I may have (old) passkeys saved on my device. (edit: found https://github.com/Kunzisoft/KeePassDX/issues/2300 about this apparently)
* Is not the WebAuthn API the same? So then you may only want to fallback even to some hardware security token, for obvious reasons this is yet another ecurity-relevant decision users need to be able to make when deciding where to store the passkey (and also seeing how secure/relevant that service may be).

Note in my case I use microG, so IIRC this would come up otherwise and ask me whether I want to use biometric unlock or some security key to register/select.

### Describe the solution you'd like:

Just as on the desktop, we should strive for feature parity here. It should in case fallback to the system service for Passkeys and/or offer a decision to choose which service you want to use.

### Describe alternatives you've considered:

AFAIK you can disable Passkey integration in the settings currently for KeePassDX to achieve the same. That is, however, very clumsy as you need to remember to enable it again etc. etc.

### Additional context:

I guess https://github.com/Kunzisoft/KeePassDX/issues/2300 is about the same but it is just a symptom for this problem here.

Start by reviewing KeePassDX's existing Passkey integration settings and the Android system service/WebAuthn behavior described in the issue, then compare the related context in issue #2300. Done means users can choose or fall back to the system passkey service without repeatedly changing the integration setting, with the storage and retrieval behavior clearly defined.

Written by the indexing model from the issue text.