fix: 导出文档时把 mermaid mindmap 转成 flowchart LR，外部阅读器清晰可读

- 老版 mermaid 渲染 mindmap 是根居中放射状、粗彩线，外部阅读器里很难读
- 导出接口现把 ```mermaid mindmap 块转换为 flowchart LR 树状图，网页样式不受影响
- 支持 root(( )) 关键字、形状符号、::icon() 与行尾注释的清理

Co-Authored-By: Claude Code <noreply@anthropic.com>

backend/app/api/chat.py

@@ -1,5 +1,6 @@
 import json
 import logging
+import re
 from fastapi import APIRouter, Depends, HTTPException, Query
 from fastapi.responses import StreamingResponse
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -368,7 +369,9 @@ async def export_conversation(conv_id: str, fmt: str = "markdown", db: AsyncSess
     lines = [f"# {conv.title or '对话'}", f"书籍: {book.title if book else '未知'}\n"]
     for m in msgs:
         role_label = "🧑 用户" if m.role == "user" else "🤖 AI"
-        lines.append(f"## {role_label}\n\n{m.content}\n")
+        # 把 mermaid mindmap 转成 flowchart LR：老版阅读器渲染 mindmap 是放射状粗线，很难读
+        content = _transform_mindmaps(m.content)
+        lines.append(f"## {role_label}\n\n{content}\n")
         if m.citations:
             try:
                 citations = json.loads(m.citations)
@@ -379,6 +382,81 @@ async def export_conversation(conv_id: str, fmt: str = "markdown", db: AsyncSess
     return {"content": "\n".join(lines), "format": fmt, "title": conv.title}
 
 
+_FENCE_RE = re.compile(r"```(mermaid)?\s*\n(.*?)\n```", re.DOTALL)
+
+
+def _transform_mindmaps(text: str) -> str:
+    """把 markdown 里的 ```mermaid mindmap 块转换成 flowchart LR（清晰左→右树状）。"""
+    def _repl(m: re.Match) -> str:
+        body = m.group(2)
+        if body.lstrip().startswith("mindmap"):
+            return f"```mermaid\n{_mindmap_to_flowchart(body)}\n```"
+        return m.group(0)
+
+    return _FENCE_RE.sub(_repl, text)
+
+
+def _mindmap_to_flowchart(body: str) -> str:
+    """mindmap（缩进式）→ flowchart LR（节点 + 边）。
+
+    mindmap:
+      root((主题))
+        一级分支
+          二级分支
+    → flowchart LR:
+      n1["主题"]
+      n1 --> n2["一级分支"]
+      n2 --> n3["二级分支"]
+    """
+    lines = []
+    for raw in body.splitlines():
+        s = raw.strip()
+        if not s or s == "mindmap":
+            continue
+        lines.append(raw)
+    if not lines:
+        return "flowchart LR\n  n1[空思维导图]"
+
+    out = ["flowchart LR"]
+    stack: list[tuple[int, str]] = []  # (缩进长度, 节点 id)
+    seq = 0
+
+    def _new_id() -> str:
+        nonlocal seq
+        seq += 1
+        return f"n{seq}"
+
+    for raw in lines:
+        m = re.match(r"[ \t]*", raw)
+        indent_len = len(m.group(0).replace("\t", "  "))
+        text = raw[m.end():].strip()
+        # 去掉 ::icon(...) 与行尾 # 注释
+        text = re.sub(r"::icon\([^)]*\)", "", text)
+        text = re.split(r"\s+#\s*", text)[0].strip()
+        # 去掉 mindmap 的 root 关键字：root((主题)) 里的 root 只是根标记，标签在形状符号内
+        if text.startswith("root") and len(text) > 4:
+            rest = text[4:].lstrip()
+            if rest and rest[0] in "([{":
+                text = rest
+        # 去掉 mindmap 形状符号包裹：(()) / [[ ]] / [ ] / ( ) / { }
+        for open_, close in (("((", "))"), ("[[", "]]"), ("[", "]"), ("(", ")"), ("{", "}")):
+            if text.startswith(open_) and text.endswith(close):
+                text = text[len(open_):-len(close)].strip()
+                break
+        label = text.strip() or _new_id()
+
+        while stack and stack[-1][0] >= indent_len:
+            stack.pop()
+        nid = _new_id()
+        safe = label.replace("\\", "\\\\").replace('"', '\\"').replace("\n", " ")
+        out.append(f'  {nid}["{safe}"]')
+        if stack:
+            out.append(f"  {stack[-1][1]} --> {nid}")
+        stack.append((indent_len, nid))
+
+    return "\n".join(out)
+
+
 def _msg_to_out(m: Message) -> MessageOut:
     citations = None
     if m.citations: