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/activeElement.js

@@ -0,0 +1,25 @@
+import ownerDocument from './ownerDocument';
+/**
+ * Returns the actively focused element safely.
+ *
+ * @param doc the document to check
+ */
+
+export default function activeElement(doc) {
+  if (doc === void 0) {
+    doc = ownerDocument();
+  }
+
+  // Support: IE 9 only
+  // IE9 throws an "Unspecified error" accessing document.activeElement from an <iframe>
+  try {
+    var active = doc.activeElement; // IE11 returns a seemingly empty object in some cases when accessing
+    // document.activeElement from an <iframe>
+
+    if (!active || !active.nodeName) return null;
+    return active;
+  } catch (e) {
+    /* ie throws if no active element */
+    return doc.body;
+  }
+}