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/closest.d.ts

@@ -0,0 +1,8 @@
+/**
+ * Returns the closest parent element that matches a given selector.
+ *
+ * @param node the reference element
+ * @param selector the selector to match
+ * @param stopAt stop traversing when this element is found
+ */
+export default function closest(node: Element, selector: string, stopAt?: Element): Element | null;