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.test.ts

@@ -0,0 +1,59 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { useEscapeKey } from '.';
+
+function dispatchEscape() {
+  globalThis.dispatchEvent(new KeyboardEvent('keydown', { key: 'Escape' }));
+}
+
+describe(useEscapeKey, () => {
+  afterEach(() => {
+    // Ensure no lingering subscribers between tests
+  });
+
+  it('fires handler on Escape', () => {
+    const h = vi.fn();
+    const stop = useEscapeKey(h);
+
+    dispatchEscape();
+    expect(h).toHaveBeenCalledTimes(1);
+
+    stop();
+  });
+
+  it('ignores non-Escape keys', () => {
+    const h = vi.fn();
+    const stop = useEscapeKey(h);
+
+    globalThis.dispatchEvent(new KeyboardEvent('keydown', { key: 'Enter' }));
+    expect(h).not.toHaveBeenCalled();
+
+    stop();
+  });
+
+  it('only topmost handler fires when multiple are stacked', () => {
+    const bottom = vi.fn();
+    const top = vi.fn();
+
+    const stopBottom = useEscapeKey(bottom);
+    const stopTop = useEscapeKey(top);
+
+    dispatchEscape();
+    expect(top).toHaveBeenCalledTimes(1);
+    expect(bottom).not.toHaveBeenCalled();
+
+    stopTop();
+    dispatchEscape();
+    expect(bottom).toHaveBeenCalledTimes(1);
+
+    stopBottom();
+  });
+
+  it('stop handle unsubscribes the listener', () => {
+    const h = vi.fn();
+    const stop = useEscapeKey(h);
+    stop();
+
+    dispatchEscape();
+    expect(h).not.toHaveBeenCalled();
+  });
+});