ci: add Lighthouse PWA + mobile a11y gate

Lighthouse CI runs on every PR against the build, with the mobile
emulation preset and these minimum scores:
  - Performance >= 0.9
  - Accessibility >= 0.95
  - PWA category (installable manifest, service worker, themed
    omnibox) must all pass

The audit runs against the three new mobile-first admin pages
(/admin/v2/orders, /admin/v2/stops, /admin/v2/products) once they
land. Until then, the audit will fail on missing routes; that's
intentional — it's the gate that PRs 3-5 must clear.

Note: package-lock.json is gitignored in this repo (per the existing
deploy workflow setup), so it is not committed alongside the
@lhci/cli dep addition.

.gitea/workflows/pwa-audit.yml

@@ -0,0 +1,34 @@
+name: PWA + Mobile A11y Audit
+
+on:
+  pull_request:
+
+jobs:
+  lighthouse:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build
+        run: npm run build
+
+      - name: Start server
+        run: npm run start &
+        env:
+          PORT: 3000
+
+      - name: Wait for server
+        run: sleep 10
+
+      - name: Run Lighthouse CI
+        run: npx lhci autorun