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>

components/login/Tab.module.css

@@ -0,0 +1,32 @@
+.list {
+  position: relative;
+  margin-bottom: var(--mantine-spacing-md);
+}
+
+.indicator {
+  border-bottom: 2px solid var(--mantine-color-blue-6);
+  top: 2px;
+
+  @mixin dark {
+    border-color: var(--mantine-color-blue-4);
+  }
+}
+
+.tab {
+  z-index: 1;
+  font-weight: 500;
+  transition: color 100ms ease;
+  color: var(--mantine-color-gray-7);
+
+  &[data-active] {
+    color: var(--mantine-color-black);
+  }
+
+  @mixin dark {
+    color: var(--mantine-color-dark-1);
+
+    &[data-active] {
+      color: var(--mantine-color-white);
+    }
+  }
+}