Document testing workflows and Node requirements in the frontend README

[collinsadi]

frontend/README.md says Node 18+ while package.json pins engines: { node: "20.x" }, and the README covers neither the vitest suites, the Playwright e2e workflow, nor the accessibility audit.

Update the README to match the toolchain and document how to run every test type.

Acceptance criteria:

• Node guidance matches the engines field and CI matrix.
• Unit, e2e, and a11y runs each have documented commands.
• The snapshot-update flow for visual regression is linked.