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/lib/lottery-api-proxy.ts

@@ -0,0 +1,69 @@
+import { type NextRequest, NextResponse } from "next/server";
+
+import { lotteryApiOrigin } from "@/lib/lottery-api-base";
+
+const HOP_BY_HOP = new Set([
+  "connection",
+  "keep-alive",
+  "proxy-authenticate",
+  "proxy-authorization",
+  "te",
+  "trailers",
+  "transfer-encoding",
+  "upgrade",
+]);
+
+/** fetch 会解压 gzip，须去掉这些头，否则浏览器 ERR_CONTENT_DECODING_FAILED */
+const STRIP_RESPONSE_HEADERS = [
+  ...HOP_BY_HOP,
+  "content-encoding",
+  "content-length",
+];
+
+/** 浏览器请求同源 `/api/*`，Next 转发到 `LOTTERY_API_UPSTREAM`（本地/线上同一套）。 */
+export async function proxyLotteryApi(
+  request: NextRequest,
+  pathSegments: string[],
+): Promise<NextResponse> {
+  const path = pathSegments.join("/");
+  const target = `${lotteryApiOrigin()}/api/${path}${request.nextUrl.search}`;
+
+  const headers = new Headers(request.headers);
+  headers.delete("host");
+  headers.delete("connection");
+  headers.delete("accept-encoding");
+
+  const init: RequestInit = {
+    method: request.method,
+    headers,
+    redirect: "manual",
+  };
+
+  if (request.method !== "GET" && request.method !== "HEAD") {
+    init.body = await request.arrayBuffer();
+  }
+
+  let upstream: Response;
+  try {
+    upstream = await fetch(target, init);
+  } catch (error: unknown) {
+    return NextResponse.json(
+      {
+        msg: "Upstream Laravel unreachable",
+        target,
+        error: error instanceof Error ? error.message : "Unknown error",
+      },
+      { status: 502 },
+    );
+  }
+  const responseHeaders = new Headers(upstream.headers);
+  for (const name of STRIP_RESPONSE_HEADERS) {
+    responseHeaders.delete(name);
+  }
+
+  return new NextResponse(upstream.body, {
+    status: upstream.status,
+    statusText: upstream.statusText,
+    headers: responseHeaders,
+  });
+}