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/math/mean.d.ts

@@ -0,0 +1,17 @@
+//#region src/math/mean.d.ts
+/**
+ * Calculates the average of an array of numbers.
+ *
+ * If the array is empty, this function returns `NaN`.
+ *
+ * @param nums - An array of numbers to calculate the average.
+ * @returns The average of all the numbers in the array.
+ *
+ * @example
+ * const numbers = [1, 2, 3, 4, 5];
+ * const result = mean(numbers);
+ * // result will be 3
+ */
+declare function mean(nums: readonly number[]): number;
+//#endregion
+export { mean };