refactor(env, api, i18n): 更新环境配置与API代理逻辑

修改 .env.example，优化API配置说明并明确线上环境要求。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API代理逻辑，移除开发代理文件，简化代码结构，提升可维护性。

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 proxy is not enabled: make sure /api is forwarded to Laravel",
+  "apiBaseMissingToast": "API proxy is not enabled: check LOTTERY_API_UPSTREAM points 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 proxy सक्षम छैन: /api Laravel मा forward भएको छ कि छैन जाँच गर्नुहोस्",
+  "apiBaseMissingToast": "API proxy सक्षम छैन: LOTTERY_API_UPSTREAM Laravel तर्फ 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": "API 代理未启用：请确认宝塔或本地 Next 已转发 /api 到 Laravel",
+  "apiBaseMissingToast": "API 代理未启用：请检查 LOTTERY_API_UPSTREAM 是否指向 Laravel",
   "captchaRequired": "请先刷新验证码",
   "welcome": "欢迎，{{name}}",
   "networkFailed": "网络请求失败",