Add login gate, calculation history, and AI markdown download.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/ai/client-stream.ts

@@ -20,7 +20,7 @@ function parseApiError(text: string, status: number): string {
 export async function streamAiCompletion(
   body: AiRequestBody,
   onUpdate: (text: string) => void,
-): Promise<void> {
+): Promise<string> {
   const res = await fetch("/api/ai", {
     method: "POST",
     cache: "no-store",
@@ -55,4 +55,6 @@ export async function streamAiCompletion(
   if (!text.trim()) {
     throw new Error("AI 返回内容为空，请检查模型配置或稍后重试");
   }
+
+  return decodeHexByteEscapes(text);
 }