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/browser/useDocumentReadyState/index.ts

@@ -0,0 +1,67 @@
+import { shallowRef } from 'vue';
+import type { ShallowRef } from 'vue';
+import { defaultDocument } from '@/types';
+import type { ConfigurableDocument } from '@/types';
+import { useEventListener } from '@/composables/browser/useEventListener';
+
+export interface UseDocumentReadyStateOptions extends ConfigurableDocument {
+  /**
+   * Called whenever `document.readyState` changes, receiving the new state,
+   * the previous state, and the originating `readystatechange` event.
+   *
+   * @default undefined
+   */
+  onChange?: (
+    state: DocumentReadyState,
+    previous: DocumentReadyState,
+    event: Event,
+  ) => void;
+}
+
+export type UseDocumentReadyStateReturn = ShallowRef<DocumentReadyState>;
+
+/**
+ * @name useDocumentReadyState
+ * @category Browser
+ * @description Reactive `document.readyState` (`loading` | `interactive` | `complete`), updated on `readystatechange`.
+ *
+ * @param {UseDocumentReadyStateOptions} [options={}] Options (custom `document`, `onChange` callback)
+ * @returns {UseDocumentReadyStateReturn} The current document ready state
+ *
+ * @example
+ * const readyState = useDocumentReadyState();
+ * watch(readyState, (state) => {
+ *   if (state === 'complete') runAfterLoad();
+ * });
+ *
+ * @example
+ * useDocumentReadyState({
+ *   onChange: (state) => {
+ *     if (state === 'interactive') hydrate();
+ *   },
+ * });
+ *
+ * @since 0.0.15
+ */
+export function useDocumentReadyState(
+  options: UseDocumentReadyStateOptions = {},
+): UseDocumentReadyStateReturn {
+  const { document = defaultDocument, onChange } = options;
+
+  const readyState = shallowRef<DocumentReadyState>(document?.readyState ?? 'loading');
+
+  if (document) {
+    useEventListener(document, 'readystatechange', (event) => {
+      const previous = readyState.value;
+      const state = document.readyState;
+
+      if (state === previous)
+        return;
+
+      readyState.value = state;
+      onChange?.(state, previous, event);
+    }, { passive: true });
+  }
+
+  return readyState;
+}