refactor: simplify thinking process UI and reduce noise

- Remove redundant status steps (understanding, preparing, generating)
  from SSE stream — only emit retrieved docs and reasoning content
- ThinkingProcess: only show when there's actual reasoning or docs
- Collapse header shows concise state: thinking count or doc count
- Clean up unused icon imports

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

backend/src/rag/conversation_chains.py

@@ -85,16 +85,8 @@ class ConversationChain:
         """流式调用（包含思考过程，捕获推理模型的真实推理内容）"""
         import time
 
-        # 思考阶段
         start_time = time.time()
-        yield {"type": "thinking", "stage": "understanding", "message": "正在理解问题..."}
-
-        # 准备历史
         history_messages = self._format_history(chat_history or [])
-        history_count = len([m for m in (chat_history or []) if m["role"] == "user"])
-        yield {"type": "thinking", "stage": "preparing", "message": f"加载对话上下文（{history_count} 轮历史）..." if history_count > 0 else "准备生成回答..."}
-
-        yield {"type": "thinking", "stage": "generating", "message": "正在生成回答..."}
 
         # 直接使用原始 OpenAI SDK 捕获推理内容
         prompt_value = await self.prompt.ainvoke({