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/usePrevious/index.ts

@@ -0,0 +1,49 @@
+import { shallowReadonly, shallowRef, toRef, watch } from 'vue';
+import type { MaybeRefOrGetter, ShallowRef, WatchOptions } from 'vue';
+
+export type UsePreviousOptions = Pick<WatchOptions, 'deep' | 'flush'>;
+
+/**
+ * @name usePrevious
+ * @category Reactivity
+ * @description Track the previous value of a ref, getter, or reactive source.
+ *
+ * @param {MaybeRefOrGetter<T>} value The source value to track
+ * @param {T} [initialValue] The initial previous value, or an options object
+ * @param {UsePreviousOptions} [options={}] Watch options (`deep`, `flush`)
+ * @returns {Readonly<ShallowRef<T | undefined>>} The previous value of the source
+ *
+ * @example
+ * const count = ref(0);
+ * const prev = usePrevious(count);
+ * count.value = 1; // prev.value === 0
+ *
+ * @example
+ * const count = ref(0);
+ * const prev = usePrevious(count, -1); // prev.value === -1 until count changes
+ *
+ * @example
+ * const state = reactive({ n: 1 });
+ * const prev = usePrevious(() => ({ ...state }), undefined, { deep: true });
+ *
+ * @since 0.0.15
+ */
+export function usePrevious<T>(value: MaybeRefOrGetter<T>, initialValue: T, options?: UsePreviousOptions): Readonly<ShallowRef<T>>;
+export function usePrevious<T>(value: MaybeRefOrGetter<T>, initialValue?: undefined, options?: UsePreviousOptions): Readonly<ShallowRef<T | undefined>>;
+export function usePrevious<T>(
+  value: MaybeRefOrGetter<T>,
+  initialValue?: T,
+  options: UsePreviousOptions = {},
+): Readonly<ShallowRef<T | undefined>> {
+  const previous = shallowRef<T | undefined>(initialValue);
+
+  watch(
+    toRef(value),
+    (_, oldValue) => {
+      previous.value = oldValue;
+    },
+    { flush: options.flush ?? 'sync', deep: options.deep },
+  );
+
+  return shallowReadonly(previous);
+}