build(deps-dev): bump vitest ecosystem to v4 and happy-dom to v20

[atomantic]

Summary

• Bumps vitest from 3.2.4 to 4.0.18
• Bumps @vitest/coverage-v8 from 3.2.4 to 4.0.18
• Bumps happy-dom from 16.8.1 to 20.7.0

These packages must be upgraded together since vitest and @vitest/coverage-v8 need matching major versions.

Closes #16, closes #18, closes #22

[Copilot]

Pull request overview

Updates the project’s dev tooling dependencies to the latest Vitest v4 ecosystem and Happy DOM v20 to keep the test stack current and aligned across related packages.

Changes:

• Bump vitest to 4.0.18
• Bump @vitest/coverage-v8 to 4.0.18
• Bump happy-dom to 20.7.0

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 2 comments.

File	Description
package.json	Updates devDependency versions for Vitest + coverage provider + Happy DOM.
package-lock.json	Refreshes lockfile to reflect the updated Vitest/coverage/happy-dom dependency graph.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

These upgrades introduce a Node.js >=20 requirement (happy-dom 20.x requires >=20 and Vitest 4 also drops Node 18). The repo currently doesn’t declare an engines.node constraint in package.json and the docs state Node 18+, so installs on Node 18 will now fail/confuse users. Consider adding an explicit "engines": {"node": ">=20"} (and updating the prerequisite docs accordingly) as part of this dependency bump.

[Copilot]

Vitest packages tend to require exact version alignment (e.g., @vitest/coverage-v8 declares a peer dependency on vitest 4.0.18). Using caret ranges here can allow a resolver to pick vitest 4.0.x that doesn’t have a matching @vitest/coverage-v8 4.0.x published yet, causing peer-dependency install failures. Consider pinning vitest and @vitest/coverage-v8 to the exact same version (no ^) to avoid that class of breakage.