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/useEscapeKey/index.ts

@@ -0,0 +1,60 @@
+import type { VoidFunction } from '@robonen/stdlib';
+import { noop } from '@robonen/stdlib';
+import { useEventListener } from '@/composables/browser/useEventListener';
+import { tryOnScopeDispose } from '@/composables/lifecycle/tryOnScopeDispose';
+import { defaultWindow } from '@/types';
+
+type EscapeListener = (event: KeyboardEvent) => void;
+
+// Module-scoped stack: only the topmost non-paused layer handles Escape so that
+// nested dismissables behave correctly (top-most dialog closes first).
+const stack: EscapeListener[] = [];
+let installed = false;
+let cleanup: VoidFunction = noop;
+
+function install() {
+  if (installed || !defaultWindow) return;
+  installed = true;
+
+  cleanup = useEventListener(defaultWindow, 'keydown', (event: KeyboardEvent) => {
+    if (event.key !== 'Escape') return;
+
+    const top = stack.at(-1);
+    top?.(event);
+  }, { capture: true });
+}
+
+function uninstall() {
+  if (!installed || stack.length > 0) return;
+  installed = false;
+  cleanup();
+  cleanup = noop;
+}
+
+/**
+ * @name useEscapeKey
+ * @category Browser
+ * @description Register a callback for the topmost Escape keydown. Uses an internal
+ * stack so that nested layers (e.g. nested Dialogs) dismiss in the correct order —
+ * only the most recently-registered listener fires for a given keydown.
+ *
+ * @param {(event: KeyboardEvent) => void} handler Callback invoked on the topmost Escape
+ * @returns {VoidFunction} Stop handle that removes the subscription
+ *
+ * @since 0.0.14
+ */
+export function useEscapeKey(handler: EscapeListener): VoidFunction {
+  if (!defaultWindow) return noop;
+
+  install();
+  stack.push(handler);
+
+  const stop = () => {
+    const i = stack.lastIndexOf(handler);
+    if (i !== -1) stack.splice(i, 1);
+    uninstall();
+  };
+
+  tryOnScopeDispose(stop);
+  return stop;
+}