webui: Add test for sketch-app-shell component

Created a basic test for the top-level Sketch App Shell component with API mocking. Reorganized mock data to be shared between tests and demo.

Co-Authored-By: sketch
diff --git a/webui/src/web-components/demo/mocks/handlers.ts b/webui/src/web-components/demo/mocks/handlers.ts
index 21bacd7..c05a6c4 100644
--- a/webui/src/web-components/demo/mocks/handlers.ts
+++ b/webui/src/web-components/demo/mocks/handlers.ts
@@ -1,5 +1,5 @@
 import { http, HttpResponse, delay } from "msw";
-import { initialState, initialMessages } from "./fixtures/dummy";
+import { initialState, initialMessages } from "../../../fixtures/dummy";
 
 // Mock state updates for long-polling simulation
 let currentState = { ...initialState };