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>

global.d.ts

@@ -0,0 +1,8 @@
+declare module '*.png' {
+  const value: { src: string }
+  export default value
+}
+declare module '*.module.css' {
+  const classes: Record<string, string>
+  export default classes
+}