test: fewer e2e tests, more unit and component tests

[hamzahalq]

The e2e suite had grown to 155 tests, and half of them were mapping-engine or single-page checks driven through a browser. This moves each check to the fastest layer that can see it.

• e2e: 155 → 36 tests (whole journeys plus layout, drag and drop, and two-tab checks), 5.5 → ~1 min.
• Component tests (new): pages render in jsdom against a mock network (React Testing Library + MSW, src/__tests__/support). A request with no handler fails the test.
• Unit/integration: added where an e2e spec was the only cover (last-admin guard, role rules, read guards, settings catalog, cron validation, CSV target options, transform argument names).
• CI: new PR check runs tsc -b and yarn test. Nothing ran the frontend tests before.
• CONTRIBUTING.md says which layer a new test goes in.

One commit per step. Local results: 428 frontend tests pass; backend unit 539/539; e2e 34/36. The two failures are a 2px scheduled-jobs overflow caused by local data, not this change. Integration failures locally are adapter packages not built on this machine, plus three AuditTrailTests that fail on r10.0 too.