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。

web/src/assets/styles/views/travel-request-detail-view.css

@@ -691,41 +691,6 @@
   gap: 8px;
 }
 
-.leader-approval-card {
-  border-color: rgba(var(--theme-primary-rgb), .18);
-  background: linear-gradient(180deg, #ffffff 0%, var(--theme-primary-soft) 100%);
-}
-
-.leader-approval-card textarea {
-  min-height: 96px;
-  background: #fff;
-  color: #0f172a;
-}
-
-.leader-approval-card textarea:focus {
-  outline: 0;
-  border-color: rgba(var(--theme-primary-rgb), .5);
-  box-shadow: 0 0 0 3px var(--theme-focus-ring);
-}
-
-.leader-opinion-meta {
-  display: flex;
-  align-items: center;
-  justify-content: space-between;
-  gap: 12px;
-  margin-top: 8px;
-  color: #64748b;
-  font-size: 12px;
-  line-height: 1.5;
-}
-
-.leader-opinion-meta strong {
-  flex: 0 0 auto;
-  color: var(--theme-primary-active);
-  font-weight: 850;
-}
-
-
 .application-leader-opinion {
   display: grid;
   gap: 10px;
@@ -763,14 +728,103 @@
   font-size: 14px;
 }
 
-.inline-leader-opinion {
-  padding: 0;
-  border: 0;
-  background: transparent;
+.application-leader-opinion-timeline {
+  position: relative;
+  display: grid;
+  gap: 10px;
+  padding-left: 18px;
 }
 
-.application-leader-opinion-display {
-  min-height: 64px;
+.application-leader-opinion-timeline::before {
+  content: "";
+  position: absolute;
+  top: 6px;
+  bottom: 6px;
+  left: 5px;
+  width: 1px;
+  background: #dbe4ee;
+}
+
+.application-leader-opinion-event {
+  position: relative;
+  display: grid;
+  gap: 8px;
+  padding: 12px 14px;
+  border: 1px solid #dbe4ee;
+  border-radius: 8px;
+  background: #ffffff;
+}
+
+.application-leader-opinion-event::before {
+  content: "";
+  position: absolute;
+  top: 17px;
+  left: -18px;
+  width: 9px;
+  height: 9px;
+  border: 2px solid #ffffff;
+  border-radius: 999px;
+  background: var(--theme-primary, #3a7ca5);
+  box-shadow: 0 0 0 1px rgba(var(--theme-primary-rgb, 58, 124, 165), .34);
+}
+
+.application-leader-opinion-event.danger::before {
+  background: #dc2626;
+  box-shadow: 0 0 0 1px rgba(220, 38, 38, .32);
+}
+
+.application-leader-opinion-event.success::before {
+  background: #16a34a;
+  box-shadow: 0 0 0 1px rgba(22, 163, 74, .32);
+}
+
+.application-leader-opinion-event-head {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 12px;
+}
+
+.application-leader-opinion-event-head span {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  color: #0f172a;
+  font-size: 14px;
+  font-weight: 850;
+}
+
+.application-leader-opinion-event-head i {
+  color: var(--theme-primary-active, #255b7d);
+  font-size: 16px;
+}
+
+.application-leader-opinion-event.danger .application-leader-opinion-event-head i {
+  color: #dc2626;
+}
+
+.application-leader-opinion-event.success .application-leader-opinion-event-head i {
+  color: #16a34a;
+}
+
+.application-leader-opinion-event-head time,
+.application-leader-opinion-event footer {
+  color: #64748b;
+  font-size: 12px;
+  font-weight: 720;
+}
+
+.application-leader-opinion-event p {
+  margin: 0;
+  color: #334155;
+  font-size: 13px;
+  line-height: 1.65;
+}
+
+.application-leader-opinion-event footer {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px;
 }
 
 .detail-expense-table {