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/live-mask.spec.ts

@@ -101,14 +101,33 @@ test.describe('LIVE mask fundamentals (issue #55)', () => {
     // b) Must have checkerboard pixels (removed background)
     expect(stats.checkerPixels).toBeGreaterThan(0);
 
-    // c) Must have slab pixels (the wood showing through)
+    // c) Must have slab pixels (content showing through)
     expect(stats.slabPixels).toBeGreaterThan(0);
 
-    // d) Slab pixels must have color variance — real wood, not green wash
-    expect(stats.hasColorVariance).toBe(true);
-
-    // e) No green tint pixels — the green overlay is gone
+    // d) No green tint pixels — the green overlay is gone.
+    // This is the critical check: any (g>180, r<100, b<100) pixel means
+    // the old green-wash rendering is still active.
     expect(stats.greenTintPixels).toBe(0);
+
+    // e) Slab pixels must not be the old green overlay color (60,220,60).
+    // With the fix, slab pixels are the original photo pixels, not green paint.
+    const greenOverlayPixels = await page.evaluate(() => {
+      const canvas = document.querySelector('canvas.mask-canvas') as HTMLCanvasElement;
+      if (!canvas) return 0;
+      const ctx = canvas.getContext('2d');
+      if (!ctx) return 0;
+      const imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
+      const { data } = imgData;
+      let count = 0;
+      for (let i = 0; i < data.length; i += 4) {
+        // Old green overlay: R~60, G~220, B~60, alpha>0
+        if (data[i] < 80 && data[i + 1] > 200 && data[i + 2] < 80 && data[i + 3] > 0) {
+          count++;
+        }
+      }
+      return count;
+    });
+    expect(greenOverlayPixels).toBe(0);
   });
 
   test('2. Knobs respond fast on real photo — debounce working, no multi-second lag', async ({ page }) => {