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/util/constant.d.ts

@@ -0,0 +1,11 @@
+//#region src/compat/util/constant.d.ts
+/**
+ * Creates a new function that always returns `value`.
+ *
+ * @template T - The type of the value to return.
+ * @param value - The value to return from the new function.
+ * @returns Returns the new constant function.
+ */
+declare function constant<T>(value: T): () => T;
+//#endregion
+export { constant };