test(gate-b): update Playwright assertions for cutout-on-checkerboard (#55)

- live-mask.spec.ts: replace green-overlay avgAlpha assertions with
  checkerboard + slab pixel detection; add green-overlay color rejection
  check; fix hasColorVariance false-positive with monochrome test image
- mask-ux.spec.ts: rename test to match new rendering (no green overlay)
- playwright.config.ts: set baseURL when webServer is used so page.goto('/')
  resolves correctly in local dev

Fixes: issue #55 mask preview

frontend/test/bvt/playwright.config.ts

@@ -15,7 +15,7 @@ export default defineConfig({
   use: {
     trace: 'on-first-retry',
     screenshot: 'only-on-failure',
-    baseURL: BASE_URL,
+    baseURL: BASE_URL || 'http://localhost:18080',
     // Ignore self-signed TLS certs on deployed hosts.
     ignoreHTTPSErrors: true,
   },