test: reconcile frontend testing/CI backlog (#1424-#1427)

[zeekman]

Closes #1424, closes #1425, closes #1426, closes #1427.

Context

These four issues are items 122-125 of a 125-issue "frontend rebuild backlog" that assumes a much larger app than what actually exists in frontend/ today: Design System primitives (Modal/Toast/Table), a split public/admin API client, a multi-route app, and six CI workflow files. The real frontend/ here is a single-page landing site with one API client (src/lib/api/client.ts) and one route. Per discussion, each issue was scoped down to the real codebase rather than fabricating the assumed components.

#1424 — interactions e2e suite

frontend/e2e/interactions.spec.ts already had isolated coverage for form validation, CTA, nav, scroll, and external links. Added a new isolated Status Announcements block covering the app's actual toast-equivalent — the aria-live status region announced on newsletter submit. No Modal or Table components exist in this app to test.

#1425 — axe-audit script

frontend/scripts/axe-audit.js now loops over an explicit ROUTES list (currently ['/'], with a note to source it from the router once there's more than one route) instead of a single hardcoded URL, and gates pass/fail on critical violations specifically, per the issue's acceptance criteria.

#1426 — API client contract tests

Added frontend/src/lib/api/__tests__/client.test.ts covering success-path parsing, error-message mapping (JSON error body, non-JSON fallback to statusText, generic HTTP {status} fallback), and empty/204-body handling — all via a mocked global.fetch, no real network or admin key needed. There's no separate public-client.ts in this repo (one client module serves everything), so no public-client.test.ts was added.

#1427 — wire CI back up

Reconciled the six referenced workflow files against the actual frontend/ structure:

• Generated the missing frontend/package-lock.json — accessibility.yml already assumed it for npm ci, but it never existed, so that workflow was broken.
• Added frontend/src/components/__tests__/LandingPage.keyboard.test.tsx so accessibility.yml's --testPathPattern="keyboard" job has a matching test to run (previously zero matches).
• Added the three workflow files that didn't exist yet: e2e-staging.yml, license-check.yml, dependency-scan.yml, plus .github/dependabot.yml — all scoped to frontend's actual npm/package-lock.json setup.
• Fixed playwright.config.ts to skip booting a local dev server when BASE_URL points at a staging deployment (needed for e2e-staging.yml to make sense).
• Left test.yml untouched — it's pure Rust/contracts CI and doesn't reference frontend/ at all, contrary to the issue's premise.

Notes

• Test suites were not executed for this change (skipped per request).
• npm install --package-lock-only was run to generate the lockfile; no dependency versions were otherwise changed.

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