feat(frontend): add API client, layout, and routing

- Create Axios-based API client with sources, jobs, and dashboard endpoints
- Add responsive Layout component with navigation sidebar
- Set up React Router with Dashboard, Backups, and Settings routes
- Configure TanStack Query provider with default options
- Use lucide-react for navigation icons

frontend/node_modules/dom-helpers/esm/toggleClass.js

@@ -0,0 +1,13 @@
+import addClass from './addClass';
+import hasClass from './hasClass';
+import removeClass from './removeClass';
+/**
+ * Toggles a CSS class on a given element.
+ * 
+ * @param element the element
+ * @param className the CSS class name
+ */
+
+export default function toggleClass(element, className) {
+  if (element.classList) element.classList.toggle(className);else if (hasClass(element, className)) removeClass(element, className);else addClass(element, className);
+}