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/isString.d.mts

@@ -0,0 +1,21 @@
+//#region src/predicate/isString.d.ts
+/**
+ * Checks if a given value is string.
+ *
+ * This function can also serve as a type predicate in TypeScript, narrowing the type of the argument to `string`.
+ *
+ * @param value The value to check if it is string.
+ * @returns Returns `true` if `value` is a string, else `false`.
+ *
+ * @example
+ * const value1 = 'abc';
+ * const value2 = 123;
+ * const value3 = true;
+ *
+ * console.log(isString(value1)); // true
+ * console.log(isString(value2)); // false
+ * console.log(isString(value3)); // false
+ */
+declare function isString(value: unknown): value is string;
+//#endregion
+export { isString };