test(02-02): add AppLayout test scaffold for NAV-01 through NAV-04

- Mock ResizeObserver and matchMedia for SidebarProvider compatibility
- NAV-01: verify sidebar data-sidebar element renders
- NAV-02: verify data-testid=sidebar-wordmark gradient element
- NAV-03: verify active nav item has data-active=true
- NAV-04: verify SidebarTrigger Toggle Sidebar button renders

frontend/src/components/AppLayout.test.tsx

@@ -0,0 +1,95 @@
+import { render, screen } from '@testing-library/react'
+import { describe, it, expect, vi } from 'vitest'
+import { AppLayout } from './AppLayout'
+import type { AuthContext } from '@/hooks/useAuth'
+
+// Mock ResizeObserver for sidebar tests
+globalThis.ResizeObserver = class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+} as any
+
+// Mock matchMedia for use-mobile hook used by SidebarProvider
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: vi.fn().mockImplementation((query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  })),
+})
+
+vi.mock('react-i18next', () => ({
+  useTranslation: () => ({ t: (key: string) => key }),
+}))
+
+vi.mock('react-router-dom', () => ({
+  Link: ({ children, to, ...props }: any) => (
+    <a href={to} {...props}>
+      {children}
+    </a>
+  ),
+  useLocation: () => ({ pathname: '/' }),
+}))
+
+const mockAuth: AuthContext = {
+  user: { display_name: 'Test' } as any,
+  loading: false,
+  login: vi.fn(),
+  register: vi.fn(),
+  logout: vi.fn(),
+  token: 'test',
+  refetch: vi.fn(),
+} as unknown as AuthContext
+
+describe('AppLayout', () => {
+  it('NAV-01: sidebar element renders with distinct background', () => {
+    render(
+      <AppLayout auth={mockAuth}>
+        <div>content</div>
+      </AppLayout>
+    )
+    // The sidebar renders with data-sidebar="sidebar" -- bg-sidebar token provides pastel background
+    const sidebar = document.querySelector('[data-sidebar="sidebar"]')
+    expect(sidebar).toBeInTheDocument()
+  })
+
+  it('NAV-02: gradient wordmark renders in sidebar header', () => {
+    render(
+      <AppLayout auth={mockAuth}>
+        <div>content</div>
+      </AppLayout>
+    )
+    const wordmark = screen.getByTestId('sidebar-wordmark')
+    expect(wordmark).toBeInTheDocument()
+  })
+
+  it('NAV-03: active nav item (dashboard at /) has data-active="true"', () => {
+    render(
+      <AppLayout auth={mockAuth}>
+        <div>content</div>
+      </AppLayout>
+    )
+    const dashboardLink = screen.getByRole('link', { name: /nav\.dashboard/i })
+    // SidebarMenuButton sets data-active on itself; it wraps the Link via asChild
+    const menuButton = dashboardLink.closest('[data-active]')
+    expect(menuButton).toHaveAttribute('data-active', 'true')
+  })
+
+  it('NAV-04: SidebarTrigger collapse button is rendered', () => {
+    render(
+      <AppLayout auth={mockAuth}>
+        <div>content</div>
+      </AppLayout>
+    )
+    // SidebarTrigger renders with sr-only text "Toggle Sidebar"
+    const trigger = screen.getByRole('button', { name: /toggle sidebar/i })
+    expect(trigger).toBeInTheDocument()
+  })
+})