feat: scaffold React frontend with login/register UI

frontend/src/api.ts

@@ -0,0 +1,45 @@
+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')
+}