chore: initial commit of external-app SSO consumer

Next.js sample application that integrates with the mock SSO service:
redirects to sso-mock for login, receives the one-time sso_code on its
/callback route, and exchanges it for user info to establish a session.

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

app/theme.ts

@@ -0,0 +1,15 @@
+'use client'
+
+import { createTheme, MantineColorsTuple } from '@mantine/core'
+
+const brandColors: MantineColorsTuple = [
+  '#E6F4FF', '#D9EBFF', '#B6D8FF', '#8EC2FF', '#63AAFF',
+  '#3393FF', '#0080FF', '#006EE6', '#005FC6', '#004897',
+]
+
+export const theme = createTheme({
+  primaryColor: 'brand',
+  cursorType: 'pointer',
+  colors: { brand: brandColors },
+  defaultRadius: 'md',
+})