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",

src/i18n/locales/ne/auth.json

@@ -16,7 +16,7 @@
   "submit": "लगइन",
   "submitting": "लगइन हुँदैछ…",
   "captchaLoadFailed": "क्याप्चा लोड गर्न सकिएन। API वा नेटवर्क जाँच गर्नुहोस्।",
-  "apiBaseMissingToast": "API कन्फिग गरिएको छैन: admin .env.local मा API_BASE_URL (Next proxy) वा NEXT_PUBLIC_LOTTERY_API_BASE_URL (direct) सेट गर्नुहोस्",
+  "apiBaseMissingToast": "API proxy सक्षम छैन: /api Laravel मा forward भएको छ कि छैन जाँच गर्नुहोस्",
   "captchaRequired": "पहिले क्याप्चा रिफ्रेस गर्नुहोस्",
   "welcome": "स्वागत छ, {{name}}",
   "networkFailed": "नेटवर्क अनुरोध असफल भयो",

src/i18n/locales/zh/auth.json

@@ -16,7 +16,7 @@
   "submit": "登录",
   "submitting": "登录中…",
   "captchaLoadFailed": "无法获取验证码，请检查接口或网络",
-  "apiBaseMissingToast": "未配置 API：请在管理端 .env.local 设置 API_BASE_URL（Next 反代），或设置 NEXT_PUBLIC_LOTTERY_API_BASE_URL（直连）",
+  "apiBaseMissingToast": "API 代理未启用：请确认宝塔或本地 Next 已转发 /api 到 Laravel",
   "captchaRequired": "请先刷新验证码",
   "welcome": "欢迎，{{name}}",
   "networkFailed": "网络请求失败",