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/toString.d.ts

@@ -0,0 +1,20 @@
+//#region src/compat/util/toString.d.ts
+/**
+ * Converts `value` to a string.
+ *
+ * An empty string is returned for `null` and `undefined` values.
+ * The sign of `-0` is preserved.
+ *
+ * @param value - The value to convert.
+ * @returns Returns the converted string.
+ *
+ * @example
+ * toString(null) // returns ''
+ * toString(undefined) // returns ''
+ * toString(-0) // returns '-0'
+ * toString([1, 2, -0]) // returns '1,2,-0'
+ * toString([Symbol('a'), Symbol('b')]) // returns 'Symbol(a),Symbol(b)'
+ */
+declare function toString(value: any): string;
+//#endregion
+export { toString };