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/offset.d.ts

@@ -0,0 +1,11 @@
+/**
+ * Returns the offset of a given element, including top and left positions, width and height.
+ *
+ * @param node the element
+ */
+export default function offset(node: HTMLElement): {
+    top: number;
+    left: number;
+    height: number;
+    width: number;
+};