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/prepend.d.ts

@@ -0,0 +1,7 @@
+/**
+ * Insert a given element as the first child of a parent element.
+ *
+ * @param node the element to prepend
+ * @param parent the parent element
+ */
+export default function prepend(node: Element | null, parent: Element | null): Element | null;