fix: make hash persistence unicode-safe

[Pixel998]

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• I have read the contributing guidelines.

AI acknowledgment

• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

What is the purpose of this pull request?

This PR fixes explorer state persistence when the edited code contains Unicode characters. Previously, writing the shareable URL hash could throw InvalidCharacterError because the app used btoa(...) on a string containing characters outside the Latin1 range.

Steps to reproduce

1. Replace the editor contents with code containing non-Latin-1 characters, for example:

const π = "😀";

2. Open the browser console.
3. Observe this error when the app tries to write the hash:

InvalidCharacterError: Failed to execute 'btoa' on 'Window': The string to be encoded contains characters outside of the Latin1 range.

What changes did you make? (Give an overview)

• Updated hash persistence to use a versioned UTF-8-safe format for newly generated links while preserving support for legacy links.
• Added e2e coverage.

Related Issues

I ran into this while working on eslint/markdown#640, where I noticed that emojis in the editor were no longer present after a reload.

Is there anything you'd like reviewers to focus on?

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[amareshsm]

LGTM. I’ll keep it open for another day in case anyone else wants to review.