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>

.gitignore

@@ -0,0 +1,41 @@
+# dependencies
+/node_modules
+/.pnp
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/versions
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+.pnpm-debug.log*
+
+# env files (never commit env; provide .env.example as a template instead)
+.env
+.env.*
+!.env.example
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo
+next-env.d.ts