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/whenever/index.ts

@@ -0,0 +1,60 @@
+import { nextTick, watch } from 'vue';
+import type { WatchCallback, WatchHandle, WatchOptions, WatchSource } from 'vue';
+
+type Truthy<T> = T extends false | null | undefined | 0 | '' ? never : T;
+
+export interface WheneverOptions<Immediate = boolean> extends WatchOptions<Immediate> {
+  /**
+   * Only trigger the callback once when the source becomes truthy.
+   *
+   * Overrides the `once` option inherited from `WatchOptions`.
+   *
+   * @default false
+   */
+  once?: boolean;
+}
+
+/**
+ * @name whenever
+ * @category Reactivity
+ * @description Shorthand for watching a source to be truthy. Behaves like `watch`, but the callback only fires when the resolved value is truthy.
+ *
+ * @param {WatchSource<T>} source The reactive source to watch
+ * @param {WatchCallback} cb Invoked with the truthy value, previous value, and `onCleanup`
+ * @param {WheneverOptions} [options] Watch options (`immediate`, `deep`, `flush`, `once`)
+ * @returns {WatchHandle} A handle to stop watching
+ *
+ * @example
+ * const ready = ref(false);
+ * whenever(ready, () => console.log('ready!'));
+ *
+ * @example
+ * whenever(() => count.value > 5, () => console.log('over five'), { once: true });
+ *
+ * @since 0.0.15
+ */
+export function whenever<T>(source: WatchSource<T>, cb: WatchCallback<Truthy<T>, T | undefined>, options?: WheneverOptions<true>): WatchHandle;
+export function whenever<T>(source: WatchSource<T>, cb: WatchCallback<Truthy<T>, T>, options?: WheneverOptions<false>): WatchHandle;
+export function whenever<T>(
+  source: WatchSource<T>,
+  cb: WatchCallback<Truthy<T>, T | undefined>,
+  options?: WheneverOptions,
+): WatchHandle {
+  const stop = watch(
+    source,
+    (value, oldValue, onCleanup) => {
+      if (value) {
+        if (options?.once)
+          nextTick(() => stop());
+
+        cb(value as Truthy<T>, oldValue, onCleanup);
+      }
+    },
+    {
+      ...options,
+      once: false,
+    } as WatchOptions,
+  );
+
+  return stop;
+}