feat: scaffold React frontend with login/register UI

frontend/src/App.test.tsx

@@ -0,0 +1,31 @@
+import { render, screen, waitFor } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import { describe, expect, it, vi } from 'vitest'
+import App from './App'
+import * as api from './api'
+
+vi.mock('./api')
+
+describe('App', () => {
+  it('renders the Quantumancy title', async () => {
+    vi.mocked(api.me).mockRejectedValue(new Error('not authenticated'))
+    render(<App />)
+    await waitFor(() => expect(screen.getByText('Quantumancy')).toBeInTheDocument())
+  })
+
+  it('logs in and shows the contact-established state', async () => {
+    vi.mocked(api.me).mockRejectedValue(new Error('not authenticated'))
+    vi.mocked(api.login).mockResolvedValue({ id: '1', username: 'medium1' })
+    render(<App />)
+
+    await waitFor(() => expect(screen.getByText('Quantumancy')).toBeInTheDocument())
+
+    await userEvent.type(screen.getByLabelText('Username'), 'medium1')
+    await userEvent.type(screen.getByLabelText('Password'), 'spookyspooky')
+    await userEvent.click(screen.getByRole('button', { name: 'Enter' }))
+
+    await waitFor(() =>
+      expect(screen.getByText('Contact established as medium1.')).toBeInTheDocument(),
+    )
+  })
+})