skip hydrate if ssr: false

[Rich-Harris]

Should fix sveltejs/svelte#10929

When using Svelte 5, we're always calling hydrate(...) instead of mount(...), even if the landing page was ssr = false. This PR fixes it.

Later, we can go a little bit further: by creating two entry points (one that references hydrate, one that references mount), and serving the mount entry point for ssr = false pages, we can take advantage of treeshaking to make the client bundle smaller in these cases. But it's probably not worth doing that until Svelte 5 is stable.

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