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>

package.json

@@ -0,0 +1,27 @@
+{
+  "name": "external-app",
+  "version": "0.1.0",
+  "private": true,
+  "scripts": {
+    "dev": "next dev -p 4000",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "@mantine/core": "^8.3.6",
+    "@mantine/hooks": "^8.3.6",
+    "@mantine/notifications": "8.3.18",
+    "@tabler/icons-react": "^3.35.0",
+    "next": "16.0.1",
+    "react": "19.2.0",
+    "react-dom": "19.2.0"
+  },
+  "devDependencies": {
+    "@types/node": "^20",
+    "@types/react": "^19",
+    "@types/react-dom": "^19",
+    "postcss-preset-mantine": "^1.18.0",
+    "postcss-simple-vars": "^7.0.1",
+    "typescript": "^5"
+  }
+}