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/toString.js

@@ -0,0 +1,28 @@
+var baseToString = require('./_baseToString');
+
+/**
+ * Converts `value` to a string. An empty string is returned for `null`
+ * and `undefined` values. The sign of `-0` is preserved.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category Lang
+ * @param {*} value The value to convert.
+ * @returns {string} Returns the converted string.
+ * @example
+ *
+ * _.toString(null);
+ * // => ''
+ *
+ * _.toString(-0);
+ * // => '-0'
+ *
+ * _.toString([1, 2, 3]);
+ * // => '1,2,3'
+ */
+function toString(value) {
+  return value == null ? '' : baseToString(value);
+}
+
+module.exports = toString;