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.

lighthouserc.json

@@ -0,0 +1,26 @@
+{
+  "ci": {
+    "collect": {
+      "url": [
+        "http://localhost:3000/admin/v2/orders",
+        "http://localhost:3000/admin/v2/stops",
+        "http://localhost:3000/admin/v2/products"
+      ],
+      "numberOfRuns": 1,
+      "settings": {
+        "preset": "mobile",
+        "emulatedFormFactor": "mobile"
+      }
+    },
+    "assert": {
+      "assertions": {
+        "categories:performance": ["error", { "minScore": 0.9 }],
+        "categories:accessibility": ["error", { "minScore": 0.95 }],
+        "categories:pwa": "error",
+        "installable-manifest": "error",
+        "service-worker": "error",
+        "themed-omnibox": "warn"
+      }
+    }
+  }
+}