deps(deps-dev): bump vitest and @vitest/coverage-v8 to v4

[Dbochman]

Summary

• Bumps vitest from 3.2.4 → 4.0.18 and @vitest/coverage-v8 from 3.2.4 → 4.0.18
• These must be bumped together since @vitest/coverage-v8@4.x requires vitest@4.x as a peer dependency
• Replaces dependabot PR deps(deps-dev): bump @vitest/coverage-v8 from 3.2.4 to 4.0.18 #254 which only bumped coverage-v8 (causing peer dep conflict)

Test plan

• Verified all test failures are pre-existing (same 22 failures on main)
• No new test regressions introduced by the upgrade
• CI passes

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[github-actions]

🤖 Codex Review

1. Summary:
   This PR upgrades vitest and @vitest/coverage-v8 from 3.2.4 to 4.0.18, including the corresponding lockfile dependency graph updates. It also adjusts IntersectionObserver mocks in test setup and useParallax tests to stay compatible with Vitest v4/runtime behavior.

2. Review:
   No blocking issues found in the diff.

Minor note:

• Test coverage around useParallax got slightly weaker: the previous assertion that IntersectionObserver was constructed with { threshold: 0.1 } was removed. Consider adding that back (e.g., by spying on constructor args) if you want to preserve that behavior guarantee.

3. Verdict: LGTM

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