docs: clarify $state export warning in universal reactivity tutorial

[mdmahendri]

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Why This Change Helps

• add into warning the technical limitation (JavaScript module imports)
• clearly distinguishes mutation (works) from reassignment (doesn't compile)

I have tried the warning

// shared.svelte.js
export let counter = $state(0);

// Counter.svelte
<script>
	import { counter } from './shared.svelte.js';
</script>

<button onclick={() => counter += 1}>
	clicks: {counter.count}
</button>

this will throw Error compiling Counter.svelte - Cannot assign to import

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