fix(ui/svelte): Allow class instances to be created as deriveds

[elliott-with-the-longest-name-on-github]

When creating class instances, this would throw an error:

const chat = $derived(new Chat());
chat.messages

...because the constructor was trying to modify state it didn't own. Hiding this modification (which is quite safe in reality) behind untrack fixes the issue.

[lgrammel]

There's an ESLint error that needs to be fixed (or it'll break main). Which bug does this fix? Or is the a new feature?