feat(frontend): add next-intl bilingual i18n for core paths

Wire next-intl without locale routes, add zh-CN/en catalogs, language
switcher, and migrate layout/auth/settings UI copy. Document i18n rules
in AGENTS.md and keep static export builds working.

frontend/package.json

@@ -54,10 +54,11 @@
     "date-fns": "^4.1.0",
     "embla-carousel-react": "^8.6.0",
     "input-otp": "^1.4.2",
-    "jspdf": "^3.0.4",
+    "jspdf": "4.2.1",
     "lucide-react": "^0.552.0",
     "motion": "^12.23.24",
     "next": "16.2.7",
+    "next-intl": "^4.13.3",
     "next-themes": "^0.4.6",
     "pinyin-match": "^1.2.10",
     "radix-ui": "^1.4.3",