gate-b: add mask UX Playwright tests — overlay visual + knob timing

- mask-ux.spec.ts: 4 tests — translucent overlay visual, sensitivity knob
  (38ms), edge offset knob (38ms), sheet switch (21ms)
- playwright.config.ts: allow PLAYWRIGHT_BASE_URL env override for remote testing

Evidence: mask-overlay-translucent.png shows translucent green overlay
with photo visible underneath.

Fixes: issue #55 — mask overlay usability evidence

frontend/test/bvt/playwright.config.ts

@@ -3,6 +3,8 @@ import { fileURLToPath } from 'node:url';
 import * as path from 'node:path';
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
+// Allow overriding via env for remote testing (e.g. against deployed instance).
+const BASE_URL = process.env.PLAYWRIGHT_BASE_URL || undefined;
 
 export default defineConfig({
   testDir: '.',
@@ -13,15 +15,18 @@ export default defineConfig({
   use: {
     trace: 'on-first-retry',
     screenshot: 'only-on-failure',
+    baseURL: BASE_URL,
   },
-  webServer: {
-    command: 'npx sirv build --port 18080',
-    port: 18080,
-    reuseExistingServer: true,
-    cwd: path.resolve(__dirname, '../../'),
-    stdout: 'ignore',
-    stderr: 'pipe',
-  },
+  webServer: BASE_URL
+    ? undefined
+    : {
+      command: 'npx sirv build --port 18080',
+      port: 18080,
+      reuseExistingServer: true,
+      cwd: path.resolve(__dirname, '../../'),
+      stdout: 'ignore',
+      stderr: 'pipe',
+    },
   projects: [
     {
       name: 'chromium',