remember the login data I enter there

For Opera's built-in password manager at least, you can goto the URL opera://password-manager/passwords and click "add" to manually add an entry. That might work, but not sure. I assume you'll have to add it specifically for https://192.168.1.121:14860/ and not https://192.168.1.121/, but not sure.

As for a password manager extension, I don't think extensions have access to that dialog.

so they won’t carry over to another PC using the same synchronized Opera account, right?

As far as I know, that's correct. You'd have to type thisisunsafe on each computer whether you use Opera Sync or not.

Thank you very much, it works great! Hopefully, it will still remember the settings even after turning off the PC and starting it again tomorrow.
The settings probably aren’t saved to the cloud, so they won’t carry over to another PC using the same synchronized Opera account, right?

And if I may ask one more question, please: I need Opera to remember the login data I enter there — can this be done as well? It’s a small embedded login window within the page, so it doesn’t open in a new tab or panel, and it never remembers the data. It also doesn’t autofill them even when using the Sticky Password app I have installed.

Opera One(verze: 123.0.5669.47)