make test-webkit runs the drag specs in WebKit against the test stack

WebKit takes *.localhost to loopback through GLib and ignores /etc/hosts, so
the container shares Traefik's network namespace. The webkit project only
exists with PLAYWRIGHT_WEBKIT=1, so test-frontend stays Chromium only.

frontend/playwright.config.ts

@@ -81,14 +81,22 @@ export default defineConfig({
     //   dependencies: ['setup'],
     // },
 
-    // {
-    //   name: 'webkit',
-    //   use: {
-    //     ...devices['Desktop Safari'],
-    //     storageState: 'playwright/.auth/user.json',
-    //   },
-    //   dependencies: ['setup'],
-    // },
+    /* The drag specs in WebKit, where the engines differ. Opt-in, because
+       WebKit only reaches a *.localhost stack from inside Traefik's network
+       namespace, which is what `make test-webkit` runs in. */
+    ...(process.env.PLAYWRIGHT_WEBKIT
+      ? [
+          {
+            name: 'webkit',
+            use: {
+              ...devices['Desktop Safari'],
+              storageState: 'playwright/.auth/user.json',
+            },
+            testMatch: /-drag\.spec\.ts/,
+            dependencies: ['setup'],
+          },
+        ]
+      : []),
 
     // {
     //   name: 'Mobile Safari',