fix: ensure remote form fields.set triggers reactivity

[LeeWxx]

Fixes #14643
Fixes #14650

Ensure remote form fields.set() triggers reactivity, so DOM and validation update immediately after programmatic changes.

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🦋 Changeset detected

Latest commit: b74eab1

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@sveltejs/kit	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14661-svelte.vercel.app/

[teemingc]

Really nice!

[alesvaupotic]

Much needed, thanks!