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/lodash/flattenDeep.js

@@ -0,0 +1,25 @@
+var baseFlatten = require('./_baseFlatten');
+
+/** Used as references for various `Number` constants. */
+var INFINITY = 1 / 0;
+
+/**
+ * Recursively flattens `array`.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.0.0
+ * @category Array
+ * @param {Array} array The array to flatten.
+ * @returns {Array} Returns the new flattened array.
+ * @example
+ *
+ * _.flattenDeep([1, [2, [3, [4]], 5]]);
+ * // => [1, 2, 3, 4, 5]
+ */
+function flattenDeep(array) {
+  var length = array == null ? 0 : array.length;
+  return length ? baseFlatten(array, INFINITY) : [];
+}
+
+module.exports = flattenDeep;