Option to prerender 404 page

Describe the problem

I am using the static adapter. The recommended way to generate a 404 page seems to be by using the fallback option, however, this option is meant to be used to generate a SPA, so it outputs a blank HTML page. I would like my 404 page to be prerendered.

Describe the proposed solution

I propose adding an option to adapter-static to generate a prerendered 404 page.

Alternatives considered

I tried simply creating 404/+page.svelte. There are two problems with this approach:

1. It doesn't work during development, because the server renders +error.svelte.
2. It doesn't work when serving the file from a subdirectory, because SvelteKit uses relative paths to assets.

Problem 1 can be solved by turning the 404 page into a component and rendering it in both 404/+page.svelte and +error.svelte.

Problem 2 can be solved by setting kit.paths.relative to false.

This feels quite hacky though.

Importance

nice to have

Additional Information

No response

Start by reviewing adapter-static's existing fallback behavior and the error/404 rendering paths described in the issue. Determine how a new option would fit both development rendering and deployment from a subdirectory. Done means adapter-static can emit a prerendered 404 page without requiring the proposed component duplication or relative-path workaround.