ci: add coverage reporting, bump deprecated action versions

[avinxshKD]

Fixes #3895

Both actions/checkout and actions/setup-node were still on v2/v1 (Node 12, deprecated for ages, GitHub's been warning about it in every run). Also added --coverage to the test step with a text-summary reporter and a 5% line threshold so we at least have a number to look at in CI.

Changes: Bumped deprecated GitHub Actions to v4 and enabled Jest coverage in CI with a text summary plus a minimal 5% global line threshold so coverage regressions don’t slip by silently.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #3895
• meets the standards outlined in the accessibility guidelines

[avinxshKD]

@raclim Hey kept this intentionally small, bumped deprecated Actions to v4 and turned on Jest coverage output in CI so we finally get a baseline in logs. I set the global line threshold to 5% on purpose as a guardrail (not a quality target) so this won’t break existing work while still catching accidental drops. If you’re good with this, I can follow up with a separate PR to ratchet thresholds up gradually (e.g. +5% per pass) once we’ve got stable numbers for a week or two.