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/cjs/toggleClass.d.ts

@@ -0,0 +1,7 @@
+/**
+ * Toggles a CSS class on a given element.
+ *
+ * @param element the element
+ * @param className the CSS class name
+ */
+export default function toggleClass(element: Element | SVGElement, className: string): void;