[fix] call onDestroy in SSR

[dummdidumm]

Via #6416, lifecycle hooks became noops for SvelteKit. But onDestroy is said to be called suring SSR according to the docs.

Fixes #6676

This PR goes this "fix" route of #6676 . Whether we switch that to "works as designed" for SvelteKit is up for discussion, therefore marking this PR as draft for now.

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Tests

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