feat(frontend): add initial Next.js UI app

.gitignore

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+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+.pnpm-debug.log*
+
+# env files
+.env*
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo
+next-env.d.ts
+
+# local caches
+__pycache__/
+*.pyc
+
+# generated files
+team_registration_result_*.json
+init_response.json
+
+# sensitive / high-risk local files
+scripts/
+CTF竞赛规则.md
+unified_pay.py
+.ace-tool/