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/refAutoReset/index.ts

@@ -0,0 +1,59 @@
+import { customRef, toValue } from 'vue';
+import type { MaybeRefOrGetter, Ref } from 'vue';
+import { useTimeoutFn } from '@/composables/utilities/useTimeoutFn';
+
+export type RefAutoResetReturn<T> = Ref<T>;
+
+/**
+ * @name refAutoReset
+ * @category Reactivity
+ * @description Create a ref that resets to its default value after a delay
+ * since the last write. Each set restarts the timer; reading is reactive.
+ *
+ * @param {MaybeRefOrGetter<T>} defaultValue The value the ref resets to (resolved each reset, can be reactive)
+ * @param {MaybeRefOrGetter<number>} [afterMs=10000] Delay in milliseconds before resetting (resolved on each set, can be reactive)
+ * @returns {RefAutoResetReturn<T>} A ref that auto-resets to `defaultValue`
+ *
+ * @example
+ * const message = refAutoReset('', 1000);
+ * message.value = 'Saved!'; // reverts to '' after 1000ms
+ *
+ * @example
+ * // Reactive delay and default
+ * const delay = ref(2000);
+ * const fallback = ref('idle');
+ * const status = refAutoReset(fallback, delay);
+ *
+ * @since 0.0.15
+ */
+export function refAutoReset<T>(
+  defaultValue: MaybeRefOrGetter<T>,
+  afterMs: MaybeRefOrGetter<number> = 10000,
+): RefAutoResetReturn<T> {
+  return customRef<T>((track, trigger) => {
+    let value: T = toValue(defaultValue);
+
+    const { start, stop } = useTimeoutFn(
+      () => {
+        value = toValue(defaultValue);
+        trigger();
+      },
+      afterMs,
+      { immediate: false },
+    );
+
+    return {
+      get() {
+        track();
+        return value;
+      },
+      set(newValue) {
+        value = newValue;
+        trigger();
+
+        stop();
+        start();
+      },
+    };
+  });
+}