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/flatMap.js

@@ -0,0 +1,29 @@
+var baseFlatten = require('./_baseFlatten'),
+    map = require('./map');
+
+/**
+ * Creates a flattened array of values by running each element in `collection`
+ * thru `iteratee` and flattening the mapped results. The iteratee is invoked
+ * with three arguments: (value, index|key, collection).
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category Collection
+ * @param {Array|Object} collection The collection to iterate over.
+ * @param {Function} [iteratee=_.identity] The function invoked per iteration.
+ * @returns {Array} Returns the new flattened array.
+ * @example
+ *
+ * function duplicate(n) {
+ *   return [n, n];
+ * }
+ *
+ * _.flatMap([1, 2], duplicate);
+ * // => [1, 1, 2, 2]
+ */
+function flatMap(collection, iteratee) {
+  return baseFlatten(map(collection, iteratee), 1);
+}
+
+module.exports = flatMap;