keep browser stores in always-accessible singleton

[Conduitry]

Resolves #6043.

This lets you use $app/stores everywhere on the browser, not just where you would be allowed to on the server. We don't need to worry about mixing data for different requests like we do on the server - there's only ever one set of stores at once - so we can be a lot more relaxed about this.

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🦋 Changeset detected

Latest commit: ce51908

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[dummdidumm]

Love this!