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/react-transition-group/esm/utils/SimpleSet.js

@@ -0,0 +1,31 @@
+var SimpleSet = /*#__PURE__*/function () {
+  function SimpleSet() {
+    this.v = [];
+  }
+
+  var _proto = SimpleSet.prototype;
+
+  _proto.clear = function clear() {
+    this.v.length = 0;
+  };
+
+  _proto.has = function has(k) {
+    return this.v.indexOf(k) !== -1;
+  };
+
+  _proto.add = function add(k) {
+    if (this.has(k)) return;
+    this.v.push(k);
+  };
+
+  _proto.delete = function _delete(k) {
+    var idx = this.v.indexOf(k);
+    if (idx === -1) return false;
+    this.v.splice(idx, 1);
+    return true;
+  };
+
+  return SimpleSet;
+}();
+
+export { SimpleSet as default };