feat: add Lighthouse CI accessibility enforcement to frontend workflow

[Joyyyb]

Summary

Closes the gap where frontend accessibility regressions could be merged undetected by adding an automated Lighthouse CI check that gates every PR and push to main/master.

Problem

There was no automated mechanism to enforce a minimum accessibility score on the Checkmate-Escrow frontend. Any change that degraded WCAG compliance (missing ARIA labels, insufficient colour contrast, unlabelled form controls, etc.) would slip through CI without notice, accumulating technical debt and potentially breaking assistive-technology users.

What was done

1. frontend/lighthouserc.js (new file)

Created a Lighthouse CI configuration file at the root of the frontend/ directory. Key settings:

• collect.staticDistDir: points to ./dist so that lhci serves the Vite production build locally — no live URL or test server required.
• collect.numberOfRuns: 3: runs three Lighthouse passes and takes the median score, reducing noise from single-run variance.
• assert.assertions['categories:accessibility']: set to ['error', { minScore: 0.9 }], which causes lhci autorun to exit with a non-zero status (failing the CI step) whenever the accessibility score drops below 90/100.
• upload.target: 'temporary-public-storage': uploads the full report to Google's temporary storage so reviewers can inspect detailed findings via a shareable URL without needing a dedicated LHCI server.

2. .github/workflows/frontend-accessibility.yml (new file)

Added a dedicated GitHub Actions workflow that runs the Lighthouse CI audit on every push and pull request that touches files under frontend/ or the workflow file itself (path-filtered to avoid burning CI minutes on unrelated changes).

Workflow steps:

1. actions/checkout@v4 — checks out the repository.
2. actions/setup-node@v4 (Node 20) — sets up Node with npm cache keyed on frontend/package-lock.json for fast dependency restoration.
3. npm ci — installs exact locked dependencies (no surprises).
4. npm run build — produces the ./dist static build that Lighthouse will audit (uses tsc -b && vite build per the existing package.json script).
5. npm install -g @lhci/cli@0.14.x — installs the Lighthouse CI CLI at a pinned minor version to avoid unexpected breaking changes.
6. lhci autorun --config=./lighthouserc.js — runs the audit; the step (and therefore the whole job) fails if the 90-point threshold is not met, blocking the PR from merging.
7. actions/upload-artifact@v4 — uploads the .lighthouseci/ directory as a 30-day artifact named lighthouse-accessibility-report so the full HTML report is always available for review, even when the check passes.

How it enforces the threshold

Lighthouse CI uses an assertion model: minScore: 0.9 maps to the normalised 0–1 score Lighthouse returns for each audited category. If the median score across the three runs is below 0.90, lhci autorun exits with code 1, the GitHub Actions step is marked failed, and the PR status check turns red — preventing merge under standard branch protection rules.

Files changed

• frontend/lighthouserc.js — new Lighthouse CI config
• .github/workflows/frontend-accessibility.yml — new CI workflow

Summary

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Related Issue

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What changed

• Contracts:
• Tests:
• Docs:
• Events / API / storage:

Verification

• What did you run to verify this change?
  • cargo test -p escrow
  • cargo test -p oracle
  • cargo fmt
  • cargo clippy

Checklist

• I added or updated tests covering this change.
• I updated documentation or confirmed no docs update is needed.
• I confirmed any event/API/storage changes are documented and backward-compatible.
• I manually verified the contract or CLI behavior where applicable.
• I linked this PR to the related issue.
• I included notes on any TTL or storage assumptions affecting contract state.

Notes

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Closes #907

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