fix(frontend): remove unused MockWS constructor param (#904)

The `_url` parameter on the test-only MockWS class was unused, and
ESLint's no-unused-vars rule does not honor the underscore-prefix
convention in this project's config, so the lint job failed. The mock
is installed via vi.stubGlobal at runtime, so dropping the parameter
is safe — JS still permits callers to pass a url argument.

frontend/src/components/EditorLayout/hooks/useContainerStats.test.ts

@@ -12,7 +12,7 @@ class MockWS {
   readyState = 1;
   send = vi.fn();
   close = vi.fn();
-  constructor(_url: string) { MockWS.instances.push(this); }
+  constructor() { MockWS.instances.push(this); }
   static reset() { MockWS.instances = []; }
 }
 

frontend/src/components/EditorLayout/hooks/useNotifications.test.ts

@@ -27,7 +27,7 @@ class MockWS {
   readyState = 1; // OPEN
   send = vi.fn();
   close = vi.fn();
-  constructor(_url: string) { MockWS.instances.push(this); }
+  constructor() { MockWS.instances.push(this); }
   static reset() { MockWS.instances = []; }
 }