feat: 完善审批退回流程与报销申请关联

后端优化报销单访问策略和常量定义，增强退回原因和审批状态
流转，前端完善退回对话框和审批交互组件，新增报销申请关联
模型，优化文档中心行数据和审批收件箱工具函数，增强引导
流程和会话模型，补充单元测试覆盖。

server/src/app/services/expense_claims.py

@@ -578,10 +578,26 @@ class ExpenseClaimService(
         previous_status = str(claim.status or "").strip()
         previous_stage = str(claim.approval_stage or "").strip() or "未标记审批环节"
         previous_stage_key = self._normalize_return_stage_key(previous_stage)
+        is_application_claim = self._is_expense_application_claim(claim)
+        is_direct_manager_return = previous_stage_key == "direct_manager"
+        return_event_type = (
+            "expense_application_return"
+            if is_application_claim and is_direct_manager_return
+            else "expense_claim_return"
+        )
+        return_label = (
+            "领导退回"
+            if is_application_claim and is_direct_manager_return
+            else "人工退回"
+        )
         return_reason = str(reason or "").strip()
         reason_code_payload = self._normalize_return_reason_code_payload(reason_codes)
         normalized_reason_codes = reason_code_payload["reason_codes"]
         unknown_reason_codes = reason_code_payload["unknown_reason_codes"]
+        if is_application_claim and is_direct_manager_return and not any(
+            code.startswith("application_") for code in normalized_reason_codes
+        ):
+            raise ValueError("申请单退回必须选择至少一个退单类型。")
         risk_points = [RETURN_REASON_OPTIONS[code] for code in normalized_reason_codes]
         existing_return_flags = self._collect_return_flags(claim.risk_flags_json)
         return_count = len(existing_return_flags) + 1
@@ -600,12 +616,17 @@ class ExpenseClaimService(
         message = return_reason or self._build_default_return_message(operator=operator, risk_points=risk_points)
         return_flag = {
             "source": "manual_return",
-            "event_type": "expense_claim_return",
+            "event_type": return_event_type,
             "return_event_id": str(uuid.uuid4()),
             "severity": "medium",
-            "label": "人工退回",
+            "label": return_label,
+            "node_key": "returned",
+            "node_label": "退回",
+            "approval_node": "退回",
             "message": message,
             "reason": return_reason,
+            "opinion": message,
+            "leader_opinion": message if is_application_claim and is_direct_manager_return else "",
             "reason_codes": normalized_reason_codes,
             "risk_points": risk_points,
             "operator": operator,