fix(vue): eslint/tsconfig migration + resolve type errors

@robonen/vue (toolkit): migrate to eslint flat config + composite tsconfig;
fix composable + test type errors (writable computed returns, null guards,
overload-compatible signatures, typed test helpers) — all type-level.

vue/toolkit/src/composables/reactivity/useArrayMap/index.ts

@@ -0,0 +1,24 @@
+import { computed, toValue } from 'vue';
+import type { ComputedRef, MaybeRefOrGetter } from 'vue';
+
+/**
+ * @name useArrayMap
+ * @category Reactivity
+ * @description Reactive `Array.prototype.map`.
+ *
+ * @param {MaybeRefOrGetter<MaybeRefOrGetter<T>[]>} list The source array (items can be reactive)
+ * @param {(element: T, index: number, array: T[]) => U} fn Mapper
+ * @returns {ComputedRef<U[]>} The mapped array
+ *
+ * @example
+ * const list = ref([1, 2, 3]);
+ * const doubled = useArrayMap(list, n => n * 2); // [2, 4, 6]
+ *
+ * @since 0.0.15
+ */
+export function useArrayMap<T, U = T>(
+  list: MaybeRefOrGetter<Array<MaybeRefOrGetter<T>>>,
+  fn: (element: T, index: number, array: T[]) => U,
+): ComputedRef<U[]> {
+  return computed(() => toValue(list).map(i => toValue(i)).map(fn));
+}