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>

utils/message.tsx

@@ -0,0 +1,77 @@
+import { IconCheck, IconX } from '@tabler/icons-react'
+import { notifications } from '@mantine/notifications'
+
+const getNotificationWidth = (text: string) => {
+  const length = Math.max(1, text?.length || 0)
+  const width = Math.min(Math.max(length * 14 + 80, 180), 520)
+  return `${width}px`
+}
+
+const getSharedStyles = (text: string) => {
+  const width = getNotificationWidth(text)
+  const offset = `calc((100% - ${width}) / 2)`
+
+  return {
+    width,
+    styles: {
+      root: {
+        top: 20,
+        left: offset,
+        transform: 'translateX(-75%)',
+        width,
+      },
+      icon: {
+        width: '16px',
+        height: '16px',
+      },
+    },
+  }
+}
+
+export const showSuccessMsg = (text: string, config?: object) => {
+  const { width, styles } = getSharedStyles(text)
+
+  notifications.show({
+    message: text,
+    position: 'top-center',
+    withCloseButton: false,
+    color: '#52c41a',
+    autoClose: 5000,
+    icon: <IconCheck />,
+    styles,
+    ...config,
+    style: {
+      width,
+      ...(config && 'style' in config && typeof config.style === 'object'
+        ? (config.style as object)
+        : {}),
+    },
+  })
+}
+
+export const showErrorMsg = (text: string, config?: object) => {
+  const { width, styles } = getSharedStyles(text)
+
+  notifications.show({
+    message: text,
+    position: 'top-center',
+    withCloseButton: false,
+    color: '#ff4d4f',
+    autoClose: 5000,
+    icon: <IconX size={14} />,
+    styles: {
+      ...styles,
+      root: {
+        ...styles.root,
+        transform: 'translateX(-75%)',
+      },
+    },
+    ...config,
+    style: {
+      width,
+      ...(config && 'style' in config && typeof config.style === 'object'
+        ? (config.style as object)
+        : {}),
+    },
+  })
+}