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.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest';
+import { ref } from 'vue';
+import { useArrayMap } from '.';
+
+describe(useArrayMap, () => {
+  it('maps reactively', () => {
+    const list = ref([1, 2, 3]);
+    const doubled = useArrayMap(list, n => n * 2);
+    expect(doubled.value).toEqual([2, 4, 6]);
+
+    list.value = [4, 5];
+    expect(doubled.value).toEqual([8, 10]);
+  });
+
+  it('unwraps reactive items', () => {
+    const list = [ref(1), ref(2)];
+    const mapped = useArrayMap(list, n => n + 1);
+    expect(mapped.value).toEqual([2, 3]);
+  });
+});