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.js

@@ -0,0 +1,20 @@
+import matches from './matches';
+/**
+ * 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, selector, stopAt) {
+  if (node.closest && !stopAt) node.closest(selector);
+  var nextNode = node;
+
+  do {
+    if (matches(nextNode, selector)) return nextNode;
+    nextNode = nextNode.parentElement;
+  } while (nextNode && nextNode !== stopAt && nextNode.nodeType === document.ELEMENT_NODE);
+
+  return null;
+}