refactor(env, i18n, http): 更新环境配置与错误提示信息

修改 .env.example，简化 API 配置说明并明确生产环境要求。更新多语言错误提示，确保用户在未启用 API 代理时获得清晰反馈。重构 admin-http.ts，优化 API 基础 URL 的解析逻辑，提升代码可维护性。

src/i18n/locales/en/auth.json

@@ -16,7 +16,7 @@
   "submit": "Log in",
   "submitting": "Signing in…",
   "captchaLoadFailed": "Failed to load captcha. Check the API or network.",
-  "apiBaseMissingToast": "API not configured: set API_BASE_URL in admin .env.local (Next proxy) or NEXT_PUBLIC_LOTTERY_API_BASE_URL (direct)",
+  "apiBaseMissingToast": "API proxy is not enabled: make sure /api is forwarded to Laravel",
   "captchaRequired": "Refresh the captcha first",
   "welcome": "Welcome, {{name}}",
   "networkFailed": "Network request failed",