fix(ui): fix coverage iframe url for html report preview

[hi-ogawa]

Description

Closes #4149
Closes #4760

I think the issue was simply due to coverage iframe (introduced in #3071) didn't consider ui html report use cases (introduced in #2444).

It seems that the iframe approach would naturally work as long as coverage.reportsDirectory is configured to be a subdirectory of html report output directory (for example,./html/coverage), so I'm loosing up a few checks in composables/navigation.ts to support this use case.

Ideally it would be nice to handle general cases (or at least default ./coverage directory), but I think this approach (+ adding caveat in documentation) might be acceptable to start with.

I plan to expand e2e tests for test/ui (EDIT: e2e added in ac7eb76), but as a quick test, the change can be verified by the following:

# preview mode
pnpm -C examples/basic test -- run --reporter=html --coverage.enabled --coverage.reportsDirectory=html/coverage
pnpm exec vite preview --outDir examples/basic/html


# ui watch mode
# (this should be same as before. In this case, coverage.reportsDirectory doesn't have to be a subdirectory of "html")
pnpm -C examples/basic test -- --ui --coverage.enabled --coverage.reportsDirectory=html/coverage
pnpm -C examples/basic test -- --ui --coverage.enabled

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

[netlify]

✅ Deploy Preview for fastidious-cascaron-4ded94 ready!

Name	Link
🔨 Latest commit	b872e98
🔍 Latest deploy log	https://app.netlify.com/sites/fastidious-cascaron-4ded94/deploys/6580d0fa4b0de600070c74be
😎 Deploy Preview	https://deploy-preview-4717--fastidious-cascaron-4ded94.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[jacando-timo-zimmermann]

Thank you! I would love to see this being merged soon

[jacando-timo-zimmermann]

Thank you @sheremet-va @hi-ogawa for this PR. Can I ask about the release cycles of Vitest? Is there an ETA for this?

[sheremet-va]

The plan is to release it today