[Epic] Testing & verification infrastructure

[skearnes]

Umbrella for the testing/verification build-out recommended in the triage plan (#656, §9). Today the UI has no browser automation, is Auth0-gated, and vitest runs in CI but is empty — so most bugs can only be caught by manual QA.

Children:

• Unit-test harness (jsdom/happy-dom + Testing Library + coverage)
• Dev-only no-auth path for local/E2E testing
• Playwright E2E + visual testing + local dev script
• Per-subsystem unit-test backfill PRs (converters → reducers/thunks → selectors/utils → components) — substantially complete; see the status comment below for what's covered vs. the remaining bespoke tail.

See #656 §9 for the full rationale and recommendations.

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