fix: resolve Page Builder hydration mismatch in SliceSimulator

[angeloashmore]

Resolves: prismicio/slice-machine#1768

Description

The App Router version of <SliceSimulator> has a typeof window === undefined check to conditionally read the simulator URL's state param; if rendered on the server, we don't read the parameter. The check is part of the component's render cycle, not in a useEffect.

This kind of check can lead to a React hydration error where the server renders as if there is no state param and the client renders using the actual state param value. The mismatch can throw an error.

This PR simplifies <SliceSimulator> to ignore window altogether. The change simplifies <SliceSimulator and the underlying code and eliminates a possible bug.

Checklist

• A comprehensive Linear ticket, providing sufficient context and details to facilitate the review of the PR, is linked to the PR.
• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

How to QA ¹

1. Deploy a Next.js App Router project with Prismic to Vercel
2. Open Prismic Page Builder and select a slice to preview
3. Make repeated content modifications (text changes, adding/removing slices)
4. Verify the preview updates smoothly without iframe refreshes or React errors

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Note

Low Risk
Client-only behavior is simplified and event subscriptions are stabilized; main risk is subtle changes in simulator rendering when no slices/message are present.

Overview
Removes URL state parsing from the App Router SliceSimulator render path (dropping getSlices/hasSlices) to prevent server/client hydration mismatches.

Stabilizes the StateEventType.Slices handler by using a routerRef and an empty-effect dependency array, and simplifies SliceSimulatorWrapper to always render the simulator root when no message is present (no longer conditionally hides content based on hasSlices).

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