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/attribute.js

@@ -0,0 +1,27 @@
+"use strict";
+
+exports.__esModule = true;
+exports.default = attribute;
+
+/**
+ * Gets or sets an attribute of a given element.
+ * 
+ * @param node the element
+ * @param attr the attribute to get or set
+ * @param val the attribute value
+ */
+function attribute(node, attr, val) {
+  if (node) {
+    if (typeof val === 'undefined') {
+      return node.getAttribute(attr);
+    }
+
+    if (!val && val !== '') {
+      node.removeAttribute(attr);
+    } else {
+      node.setAttribute(attr, String(val));
+    }
+  }
+}
+
+module.exports = exports["default"];