Can't save HackMD instance URL

I'm using Firefox 142.0.1 with HackMD-it 0.5.1 and I can't succeed to save `HackMD instance URL`.

When I change the field `HackMD instance URL`, a red string `Not saved` is displayed, but when I click on the `Save` button, nothing happens.

It is hence not possible to change the HackMD instant URL by the settings UI.

Reproduce the issue in Firefox 142.0.1 with HackMD-it 0.5.1 by editing the HackMD instance URL in the settings UI and selecting Save. Trace the settings save entry point and determine why the changed URL is not persisted; done means the URL saves successfully and remains changed after reopening settings.

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