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/hasClass.js

@@ -0,0 +1,10 @@
+/**
+ * Checks if a given element has a CSS class.
+ * 
+ * @param element the element
+ * @param className the CSS class name
+ */
+export default function hasClass(element, className) {
+  if (element.classList) return !!className && element.classList.contains(className);
+  return (" " + (element.className.baseVal || element.className) + " ").indexOf(" " + className + " ") !== -1;
+}