SvelteKit: Only disable SSR when building, not serving

[JReinhold]

Closes #21053

What I did

In #20804 we disabled SSR because Storybook can't be built with SSR enabled.

That broke HMR, so now we're only disabling SSR at build time, not when serving.

I'm unsure why it broke HMR, but it's not SvelteKit related, as it also breaks HMR to add that disable-SSR plugin to a plain Svelte-Vite project.

It looks like the Svelte Vite plugin is using the SSR setting to handle HMR differently, so maybe that's the culprit.

https://github.com/sveltejs/vite-plugin-svelte/blob/main/packages/vite-plugin-svelte/src/handle-hot-update.ts#L61-L65

How to test

1. Create a SvelteKit sandbox with yarn task --task sandbox --start-from auto --template svelte-kit/skeleton-ts
2. Start it and open a story.
3. Make changes to the story, and see that they are reflected in the UI.
4. Make changes to the component, and see that they are reflected in the UI.
5. Build the Storybook to ensure it can still be built.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[IanVS]

This is so bizzare, but good find. I'd lean towards considering it a vite or plugin-svelte issue, but this seems like a good workaround.

[JReinhold]

This is so bizzare, but good find. I'd lean towards considering it a vite or plugin-svelte issue, but this seems like a good workaround.

I agree, I'll see if I can build a non-Storybook repro and file an issue over there.