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/axios/lib/helpers/combineURLs.js

@@ -0,0 +1,15 @@
+'use strict';
+
+/**
+ * Creates a new URL by combining the specified URLs
+ *
+ * @param {string} baseURL The base URL
+ * @param {string} relativeURL The relative URL
+ *
+ * @returns {string} The combined URL
+ */
+export default function combineURLs(baseURL, relativeURL) {
+  return relativeURL
+    ? baseURL.replace(/\/?\/$/, '') + '/' + relativeURL.replace(/^\/+/, '')
+    : baseURL;
+}