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/_isFlattenable.js

@@ -0,0 +1,20 @@
+var Symbol = require('./_Symbol'),
+    isArguments = require('./isArguments'),
+    isArray = require('./isArray');
+
+/** Built-in value references. */
+var spreadableSymbol = Symbol ? Symbol.isConcatSpreadable : undefined;
+
+/**
+ * Checks if `value` is a flattenable `arguments` object or array.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is flattenable, else `false`.
+ */
+function isFlattenable(value) {
+  return isArray(value) || isArguments(value) ||
+    !!(spreadableSymbol && value && value[spreadableSymbol]);
+}
+
+module.exports = isFlattenable;