Add settings page for managing forecasting API key and URL via UI

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/es-toolkit/dist/compat/array/flatten.d.ts

@@ -0,0 +1,16 @@
+//#region src/compat/array/flatten.d.ts
+/**
+ * Flattens array up to depth times.
+ *
+ * @template T
+ * @param value - The array to flatten.
+ * @param depth - The maximum recursion depth.
+ * @returns Returns the new flattened array.
+ *
+ * @example
+ * flatten([1, [2, [3, [4]], 5]], 2);
+ * // => [1, 2, 3, [4], 5]
+ */
+declare function flatten<T>(value: ArrayLike<T | readonly T[]> | null | undefined): T[];
+//#endregion
+export { flatten };