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/collectElements.js

@@ -0,0 +1,12 @@
+export default function collectElements(node, direction) {
+  var nextNode = null;
+  var nodes = [];
+  nextNode = node ? node[direction] : null;
+
+  while (nextNode && nextNode.nodeType !== 9) {
+    nodes.push(nextNode);
+    nextNode = nextNode[direction] || null;
+  }
+
+  return nodes;
+}