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/querySelectorAll.js

@@ -0,0 +1,17 @@
+"use strict";
+
+exports.__esModule = true;
+exports.default = qsa;
+var toArray = Function.prototype.bind.call(Function.prototype.call, [].slice);
+/**
+ * Runs `querySelectorAll` on a given element.
+ * 
+ * @param element the element
+ * @param selector the selector
+ */
+
+function qsa(element, selector) {
+  return toArray(element.querySelectorAll(selector));
+}
+
+module.exports = exports["default"];