feat(frontend): automated accessibility testing with jest-axe (closes #132)

[Moonwalker-rgb]

What

Closes #132 — adds automated accessibility testing for the frontend using jest-axe, so axe-core violations surface in CI rather than at the aid recipient's keyboard.

Changes

Testing infrastructure

• jest-axe@^10 and axe-core@^4.12 added as devDeps
• New jest.setup.a11y.ts registers toHaveNoViolations globally via setupFilesAfterEnv
• New jest-axe.d.ts (ambient types) so strict tsc --noEmit stays clean
• New a11yTestUtils.tsx (renderAndCheckA11y helper returning a structured report) and a11y-mocks.tsx (shared mocks for next/link, next-intl, next/navigation, @stellar/freighter-api, and project hooks)
• New a11y.test.tsx — @jest-environment jsdom suite covering ErrorInline (banner + card), ActivityCenter, Navbar, ErrorBoundary, EvidenceArtifactViewer. 6 tests passing, 0 critical violations.

Component fixes (the axe violations the suite uncovered)

• Icon-only close/remove buttons in ErrorInline and ActivityCenter now carry aria-label and the inner SVG aria-hidden="true" focusable="false"
• The notification count badge in ActivityCenter is decorative (aria-hidden) with a new sr-only text node for screen readers
• EvidenceArtifactViewer filename heading promoted from <h3> to <h2> to satisfy heading-order against pages that provide an <h1>

CI

• New .github/workflows/frontend-ci.yml runs pnpm type-check, pnpm lint, and pnpm jest --ci on every PR touching app/frontend/**, using the official pnpm/action-setup and the pnpm store cache.

Lighthouse CI (documented, not gated)

• app/frontend/lighthouserc.json asserts categories:accessibility ≥ 0.9
• app/frontend/LIGHTHOUSE_CI.md explains why Lighthouse isn't part of the CI workflow yet — jest-axe already catches the same class of violations faster and with zero infrastructure overhead; Lighthouse is opt-in via pnpm dlx @lhci/cli autorun or a future scheduled workflow.

Tests

• 89 unit tests pass on the existing baseline
• 6 jest-axe tests pass with 0 critical violations
• Frontend tsc --noEmit passes
• Backend / mobile untouched by this PR

Notes

• The shared axeContext prop on renderAndCheckA11y accepts an axe RunOptions object (or undefined). Use it to scope a scan to a single element when adding page-level tests.
• Tests that depend on the <main> landmark wrap their fixture there to satisfy landmark-one-main and page-has-heading-one.
• The EvidenceArtifactViewer heading change is a deliberate conformance trade-off — if the component is later embedded under a page that already uses an <h2>, swap it for a titleHeadingLevel prop or an aria-level heading role.

[Moonwalker-rgb]

cc @Lansa-18 @maramina — recent frontend contributors. Specifically asking for review on (1) the jest-axe setup wiring in jest.config.ts, (2) the heading-order fix in EvidenceArtifactViewer.tsx since it couples the component to a specific page h1 contract, and (3) whether the lighthouserc.json should be wired up now or stay defer.