feat: 对话区域文件上传(粘贴/拖拽) + XLSX支持 + 会话切换无限循环修复

- 对话区域: st.file_uploader + 全局 paste/drop 事件监听 + sessionStorage 桥接
- 文件预览芯片: 上传后显示在对话区域，可逐文件移除
- OCR 双层解析全面接入: file_parser(文字) + ocr_extractor(字段提取)
- XLSX 解析: openpyxl 逐工作表/逐行读取
- 修复: create_session 强制写入 agent_state.session_id
- 修复: load_session_node 不再从磁盘覆盖 session_id
- 修复: 切换会话 _last_switched_to 哨兵防止无限 rerun

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

backend/session.py

@@ -38,12 +38,14 @@ def create_session(name: str = "", agent_state: Optional[dict] = None) -> dict:
     _ensure_dir()
     sid = generate_session_id()
     now = _now_iso()
+    agent_state = agent_state or {}
+    agent_state["session_id"] = sid
     data = {
         "session_id": sid,
         "session_name": name or f"新建报表 {now[:10]}",
         "created_at": now,
         "updated_at": now,
-        "agent_state": agent_state or {},
+        "agent_state": agent_state,
     }
     with open(_session_path(sid), "w", encoding="utf-8") as f:
         json.dump(data, f, ensure_ascii=False, indent=2)