chore: initial commit of sso-mock SSO login service

Next.js + Mantine mock SSO service providing account/password, phone
verify-code, and Feishu QR login. Issues a one-time sso_code that the
external app exchanges for user info.

Includes:
- Feishu callback fix: guard against duplicate/concurrent /api/login
  submissions of the same single-use authorization code (feishuLogin
  once-guard + memoized onLoginSuccess to stop effect re-fire).
- ADAPTATION_GUIDE.md: how to switch from the in-memory code store to
  the real backend auth/index + inner_get_user_info endpoints.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/page.tsx

@@ -0,0 +1,5 @@
+import { redirect } from 'next/navigation'
+
+export default function Home() {
+  redirect('/login')
+}