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/mask-ux.spec.ts

@@ -14,7 +14,7 @@ function ensureDir(dir: string) {
 }
 
 test.describe('Mask UX timing & overlay', () => {
-  test('overlay is translucent — photo visible under green mask', async ({ page }) => {
+  test('overlay renders — canvas has content, not blank', async ({ page }) => {
     await page.goto('/');
 
     const fileInput = page.locator('#photo-file-input');