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/useSyncRefs/index.ts

@@ -27,7 +27,7 @@ import { isArray } from '@robonen/stdlib';
  */
 export function useSyncRefs<T = unknown>(
   source: WatchSource<T>,
-  targets: Ref<T> | Ref<T>[],
+  targets: Ref<T> | Array<Ref<T>>,
   watchOptions: WatchOptions = {},
 ) {
   const {