feat(renderer): AppHeader with sidebar/preview toggles + theme toggle

tests/component/layout/AppHeader.test.tsx

@@ -0,0 +1,33 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { ThemeProvider } from '@/components/theme-provider';
+import { AppHeader } from '@/components/layout/AppHeader';
+import { useAppStore } from '@/stores/app-store';
+
+describe('AppHeader', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    useAppStore.setState({ sidebarVisible: true, previewVisible: true, zenMode: false });
+  });
+
+  it('renders the app title', () => {
+    render(
+      <ThemeProvider defaultTheme="dark" attribute="class">
+        <AppHeader />
+      </ThemeProvider>
+    );
+    expect(screen.getByText(/markdownconverter/i)).toBeInTheDocument();
+  });
+
+  it('toggles sidebar when sidebar button clicked', async () => {
+    render(
+      <ThemeProvider defaultTheme="dark" attribute="class">
+        <AppHeader />
+      </ThemeProvider>
+    );
+    const btn = screen.getByRole('button', { name: /toggle sidebar/i });
+    await userEvent.click(btn);
+    expect(useAppStore.getState().sidebarVisible).toBe(false);
+  });
+});