fix: `fields.value()` initial data missing

[sillvva]

fixes #14972

The issue happens when you assign form.fields to a variable and then attempt to access .value(). As a result, this results in a hydration mismatch.

const fields = test.fields;
const initial = $state({ initial: "value" });
fields.set(initial);
// On the server, this results in: {}
$inspect(fields.value());
// And this results in: { initial: "value" }
$inspect(test.fields.value());

Before: https://stackblitz.com/edit/sveltekit-308957-8ktcalyf?file=src%2Froutes%2F%2Bpage.svelte

After: https://stackblitz.com/edit/sveltekit-308957-8ktcalyf-apue36pc?file=src%2Froutes%2F%2Bpage.svelte

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