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/predicate/isError.d.mts

@@ -0,0 +1,17 @@
+//#region src/predicate/isError.d.ts
+/**
+ * Checks if `value` is an Error object.
+ *
+ * @param value The value to check.
+ * @returns Returns `true` if `value` is an Error object, `false` otherwise.
+ *
+ * @example
+ * ```typescript
+ * console.log(isError(new Error())); // true
+ * console.log(isError('Error')); // false
+ * console.log(isError({ name: 'Error', message: '' })); // false
+ * ```
+ */
+declare function isError(value: unknown): value is Error;
+//#endregion
+export { isError };