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/isNil.js

@@ -0,0 +1,25 @@
+/**
+ * Checks if `value` is `null` or `undefined`.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category Lang
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is nullish, else `false`.
+ * @example
+ *
+ * _.isNil(null);
+ * // => true
+ *
+ * _.isNil(void 0);
+ * // => true
+ *
+ * _.isNil(NaN);
+ * // => false
+ */
+function isNil(value) {
+  return value == null;
+}
+
+module.exports = isNil;