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/addClass.js

@@ -0,0 +1,11 @@
+import hasClass from './hasClass';
+/**
+ * Adds a CSS class to a given element.
+ * 
+ * @param element the element
+ * @param className the CSS class name
+ */
+
+export default function addClass(element, className) {
+  if (element.classList) element.classList.add(className);else if (!hasClass(element, className)) if (typeof element.className === 'string') element.className = element.className + " " + className;else element.setAttribute('class', (element.className && element.className.baseVal || '') + " " + className);
+}