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/flattenDepth.js

@@ -0,0 +1,33 @@
+var baseFlatten = require('./_baseFlatten'),
+    toInteger = require('./toInteger');
+
+/**
+ * Recursively flatten `array` up to `depth` times.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.4.0
+ * @category Array
+ * @param {Array} array The array to flatten.
+ * @param {number} [depth=1] The maximum recursion depth.
+ * @returns {Array} Returns the new flattened array.
+ * @example
+ *
+ * var array = [1, [2, [3, [4]], 5]];
+ *
+ * _.flattenDepth(array, 1);
+ * // => [1, 2, [3, [4]], 5]
+ *
+ * _.flattenDepth(array, 2);
+ * // => [1, 2, 3, [4], 5]
+ */
+function flattenDepth(array, depth) {
+  var length = array == null ? 0 : array.length;
+  if (!length) {
+    return [];
+  }
+  depth = depth === undefined ? 1 : toInteger(depth);
+  return baseFlatten(array, depth);
+}
+
+module.exports = flattenDepth;