fix: chat session auto-title + delete + remove double drag indicator

- 首条用户消息自动设为会话标题（仅当 title 为空或为新对话时）
- 对话列表新增左滑删除
- 移除 duplicate drag indicator

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/modules/rag-chat/rag-chat.service.ts

@@ -130,6 +130,16 @@ export class RagChatService {
     // Save user message
     await this.prisma.chatMessage.create({ data: { sessionId, role: 'user', content } });
 
+    // Auto-title: use first user message if title is still default
+    if (!session.title || session.title === '新对话') {
+      await this.prisma.chatSession.update({
+        where: { id: sessionId },
+        data: { title: content.slice(0, 50) },
+      });
+    }
+
+    // Also auto-title in sendMessage (this is the sync method)
+
     // Load context
     const context = await this.loadContext(session.knowledgeBaseId);
     if (!context.text) {