📎 Add WebSocket mock

frontend/playwright/helpers/MockRPC.js

@@ -0,0 +1,8 @@
+export const interceptRPC = (page, path, jsonFilename) =>
+  page.route(`**/api/rpc/command/${path}`, (route) =>
+    route.fulfill({
+      status: 200,
+      contentType: "application/transit+json",
+      path: `playwright/fixtures/${jsonFilename}`,
+    })
+  );