feat: add Lighthouse CI accessibility enforcement to frontend workflow

.github/workflows/frontend-accessibility.yml

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+name: Frontend Accessibility (Lighthouse CI)
+
+# Run on every push and pull request that touches the frontend or this workflow.
+on:
+  push:
+    branches: [main, master]
+    paths:
+      - 'frontend/**'
+      - '.github/workflows/frontend-accessibility.yml'
+  pull_request:
+    branches: [main, master]
+    paths:
+      - 'frontend/**'
+      - '.github/workflows/frontend-accessibility.yml'
+
+jobs:
+  lighthouse-accessibility:
+    name: Lighthouse Accessibility Audit
+    runs-on: ubuntu-latest
+
+    defaults:
+      run:
+        working-directory: frontend
+
+    steps:
+      # ── 1. Checkout ──────────────────────────────────────────────────────
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      # ── 2. Node.js setup ─────────────────────────────────────────────────
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+          cache-dependency-path: frontend/package-lock.json
+
+      # ── 3. Install dependencies ───────────────────────────────────────────
+      - name: Install dependencies
+        run: npm ci
+
+      # ── 4. Build the frontend ─────────────────────────────────────────────
+      # Lighthouse CI needs a static build to audit; `npm run build` produces
+      # the ./dist directory referenced in lighthouserc.js.
+      - name: Build frontend
+        run: npm run build
+
+      # ── 5. Install Lighthouse CI ──────────────────────────────────────────
+      - name: Install @lhci/cli
+        run: npm install -g @lhci/cli@0.14.x
+
+      # ── 6. Run Lighthouse CI ──────────────────────────────────────────────
+      # Uses frontend/lighthouserc.js which asserts categories:accessibility >= 0.90.
+      # The step fails (exit code 1) if the score drops below the threshold,
+      # blocking the PR from merging.
+      - name: Run Lighthouse CI
+        run: lhci autorun --config=./lighthouserc.js
+        env:
+          LHCI_GITHUB_APP_TOKEN: ${{ secrets.LHCI_GITHUB_APP_TOKEN }}
+
+      # ── 7. Upload Lighthouse report as artifact ───────────────────────────
+      - name: Upload Lighthouse results
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: lighthouse-accessibility-report
+          path: frontend/.lighthouseci/
+          retention-days: 30

frontend/lighthouserc.js

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+/**
+ * Lighthouse CI configuration for the Checkmate-Escrow frontend.
+ *
+ * Enforces a minimum accessibility score of 90 so that any PR that
+ * regresses the frontend's WCAG compliance is caught before it merges.
+ *
+ * @see https://github.com/GoogleChrome/lighthouse-ci/blob/main/docs/configuration.md
+ */
+
+export default {
+  ci: {
+    collect: {
+      // Build the Vite app and serve the static output locally.
+      staticDistDir: './dist',
+      // Run three passes and take the median to reduce score variance.
+      numberOfRuns: 3,
+    },
+    assert: {
+      assertions: {
+        // Fail the CI run if the accessibility score drops below 0.90 (90/100).
+        'categories:accessibility': ['error', { minScore: 0.9 }],
+      },
+    },
+    upload: {
+      // Store results as GitHub Actions artifacts — no external LHCI server needed.
+      target: 'temporary-public-storage',
+    },
+  },
+};