feat(memoir+conversation): 章节/故事最小可读字数；会话 hasUserMessage 与 UI 优化

- 后端：300 字门槛统一物化、hydrate、列表/PDF/详情；过短章节对读者隐藏
- 对话：首包前打字动画、大字模式排版、朗读/TTS 交互与布局稳定
- 首页：复用无用户消息会话；空列表「继续对话」与文案 i18n
- 章节阅读：标题进正文、封面与去重标题；阅读 Markdown 字号上调

api/app/features/conversation/service.py

@@ -146,6 +146,7 @@ class ConversationService:
             except Exception:
                 pass
             latest_message = history[-1].get("content", "")[:50] if history else None
+            has_user_message = any((msg.get("role") == "human") for msg in history)
             result.append(
                 {
                     "id": conv.id,
@@ -155,6 +156,7 @@ class ConversationService:
                     "latestMessageTime": _latest_message_time_ms(conv, history),
                     "unreadCount": 0,
                     "isDefaultAssistant": conv.summary is None,
+                    "hasUserMessage": has_user_message,
                 }
             )
         return result