feat: scaffold React frontend with login/register UI

frontend/src/App.css

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+:root {
+  color-scheme: dark;
+}
+
+body {
+  margin: 0;
+  min-height: 100vh;
+  background: #0a0a0f;
+  color: #e6e6f0;
+  font-family: Georgia, serif;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+
+.app {
+  width: min(90vw, 420px);
+  padding: 2rem;
+  text-align: center;
+}
+
+h1 {
+  font-size: 2.5rem;
+  letter-spacing: 0.1em;
+  margin-bottom: 0.25rem;
+}
+
+.tagline {
+  opacity: 0.6;
+  margin-bottom: 2rem;
+  font-style: italic;
+}
+
+form {
+  display: flex;
+  flex-direction: column;
+  gap: 1rem;
+  text-align: left;
+}
+
+label {
+  display: flex;
+  flex-direction: column;
+  gap: 0.25rem;
+  font-size: 0.9rem;
+  opacity: 0.8;
+}
+
+input {
+  background: #16161f;
+  border: 1px solid #333;
+  color: inherit;
+  padding: 0.6rem;
+  border-radius: 4px;
+  font-size: 1rem;
+}
+
+button {
+  background: #2a1f3d;
+  border: 1px solid #5a3f8f;
+  color: inherit;
+  padding: 0.6rem 1rem;
+  border-radius: 4px;
+  cursor: pointer;
+  font-size: 1rem;
+}
+
+button:hover {
+  background: #3a2a55;
+}
+
+.mode-toggle {
+  display: flex;
+  gap: 0.5rem;
+}
+
+.mode-toggle button.active {
+  background: #5a3f8f;
+}
+
+.error {
+  color: #ff6b6b;
+  font-size: 0.9rem;
+}

frontend/src/App.test.tsx

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+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(),
+    )
+  })
+})

frontend/src/App.tsx

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+import { useEffect, useState } from 'react'
+import { login, logout, me, register, type User } from './api'
+
+function App() {
+  const [user, setUser] = useState<User | null>(null)
+  const [username, setUsername] = useState('')
+  const [password, setPassword] = useState('')
+  const [mode, setMode] = useState<'login' | 'register'>('login')
+  const [error, setError] = useState<string | null>(null)
+  const [checkingSession, setCheckingSession] = useState(true)
+
+  useEffect(() => {
+    me()
+      .then(setUser)
+      .catch(() => setUser(null))
+      .finally(() => setCheckingSession(false))
+  }, [])
+
+  async function handleSubmit(event: React.FormEvent) {
+    event.preventDefault()
+    setError(null)
+    try {
+      if (mode === 'register') {
+        await register(username, password)
+      }
+      const loggedInUser = await login(username, password)
+      setUser(loggedInUser)
+    } catch (err) {
+      setError(err instanceof Error ? err.message : 'Something went wrong')
+    }
+  }
+
+  async function handleLogout() {
+    await logout()
+    setUser(null)
+  }
+
+  if (checkingSession) {
+    return (
+      <main className="app">
+        <p>Listening for a signal...</p>
+      </main>
+    )
+  }
+
+  return (
+    <main className="app">
+      <h1>Quantumancy</h1>
+      <p className="tagline">Something is listening on the other side.</p>
+
+      {user ? (
+        <section>
+          <p>Contact established as {user.username}.</p>
+          <button onClick={handleLogout}>End session</button>
+        </section>
+      ) : (
+        <form onSubmit={handleSubmit}>
+          <div className="mode-toggle">
+            <button
+              type="button"
+              className={mode === 'login' ? 'active' : ''}
+              onClick={() => setMode('login')}
+            >
+              Reconnect
+            </button>
+            <button
+              type="button"
+              className={mode === 'register' ? 'active' : ''}
+              onClick={() => setMode('register')}
+            >
+              First contact
+            </button>
+          </div>
+          <label>
+            Username
+            <input value={username} onChange={(e) => setUsername(e.target.value)} required />
+          </label>
+          <label>
+            Password
+            <input
+              type="password"
+              value={password}
+              onChange={(e) => setPassword(e.target.value)}
+              required
+              minLength={8}
+            />
+          </label>
+          {error && <p className="error">{error}</p>}
+          <button type="submit">{mode === 'login' ? 'Enter' : 'Begin'}</button>
+        </form>
+      )}
+    </main>
+  )
+}
+
+export default App

frontend/src/api.ts

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+export type User = {
+  id: string
+  username: string
+}
+
+async function request<T>(path: string, options: RequestInit = {}): Promise<T> {
+  const response = await fetch(path, {
+    ...options,
+    credentials: 'include',
+    headers: {
+      'Content-Type': 'application/json',
+      ...options.headers,
+    },
+  })
+  if (!response.ok) {
+    const body = await response.json().catch(() => ({ detail: response.statusText }))
+    throw new Error(body.detail ?? `Request failed: ${response.status}`)
+  }
+  if (response.status === 204) {
+    return undefined as T
+  }
+  return response.json() as Promise<T>
+}
+
+export function register(username: string, password: string): Promise<User> {
+  return request<User>('/auth/register', {
+    method: 'POST',
+    body: JSON.stringify({ username, password }),
+  })
+}
+
+export function login(username: string, password: string): Promise<User> {
+  return request<User>('/auth/login', {
+    method: 'POST',
+    body: JSON.stringify({ username, password }),
+  })
+}
+
+export function logout(): Promise<void> {
+  return request<void>('/auth/logout', { method: 'POST' })
+}
+
+export function me(): Promise<User> {
+  return request<User>('/auth/me')
+}

frontend/src/main.tsx

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+import { StrictMode } from 'react'
+import { createRoot } from 'react-dom/client'
+import App from './App'
+import './App.css'
+
+createRoot(document.getElementById('root')!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+)

frontend/src/setupTests.ts

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+import '@testing-library/jest-dom'