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/views/TravelRequestDetailView.vue

@@ -182,21 +182,26 @@
                   <span><i class="mdi mdi-account-tie-outline"></i>领导意见</span>
                   <strong v-if="leaderApprovalReadonlyMeta">{{ leaderApprovalReadonlyMeta }}</strong>
                 </div>
-                <div v-if="showApplicationLeaderOpinionInput" class="leader-approval-card inline-leader-opinion">
-                  <textarea
-                    v-model="leaderOpinion"
-                    maxlength="500"
-                    :required="requiresApprovalOpinion"
-                    :placeholder="approvalOpinionPlaceholder"
-                    :aria-label="approvalOpinionTitle"
-                  ></textarea>
-                  <div class="leader-opinion-meta">
-                    <span>{{ approvalOpinionHint }}</span>
-                    <strong>{{ leaderOpinion.length }}/500</strong>
-                  </div>
-                </div>
-                <div v-else class="detail-note readonly application-leader-opinion-display">
-                  <p>{{ leaderApprovalReadonlyText }}</p>
+                <div v-if="hasLeaderApprovalEvents" class="application-leader-opinion-timeline" aria-label="领导批复事件流">
+                  <article
+                    v-for="event in leaderApprovalEvents"
+                    :key="event.id"
+                    class="application-leader-opinion-event"
+                    :class="event.tone"
+                  >
+                    <div class="application-leader-opinion-event-head">
+                      <span>
+                        <i :class="event.type === 'returned' ? 'mdi mdi-arrow-u-left-top' : 'mdi mdi-check-circle-outline'"></i>
+                        {{ event.title }}
+                      </span>
+                      <time v-if="event.time">{{ event.time }}</time>
+                    </div>
+                    <p>{{ event.opinion }}</p>
+                    <footer>
+                      <span>{{ event.operator }}</span>
+                      <span v-if="event.returnCount">第 {{ event.returnCount }} 次退回</span>
+                    </footer>
+                  </article>
                 </div>
               </div>
 
@@ -475,20 +480,6 @@
               </div>
             </article>
 
-            <article v-if="showLeaderApprovalPanel" class="detail-card panel leader-approval-card">
-              <h3>{{ approvalOpinionTitle }}</h3>
-              <textarea
-                v-model="leaderOpinion"
-                maxlength="500"
-                :required="requiresApprovalOpinion"
-                :placeholder="approvalOpinionPlaceholder"
-                :aria-label="approvalOpinionTitle"
-              ></textarea>
-              <div class="leader-opinion-meta">
-                <span>{{ approvalOpinionHint }}</span>
-                <strong>{{ leaderOpinion.length }}/500</strong>
-              </div>
-            </article>
           </section>
         </div>
       </div>
@@ -774,7 +765,10 @@
       :summary-label="approvalConfirmSummaryLabel"
       :next-stage="approvalNextStage"
       :opinion-title="approvalOpinionTitle"
-      :opinion="leaderOpinion"
+      v-model:opinion="leaderOpinion"
+      :opinion-placeholder="approvalOpinionPlaceholder"
+      :opinion-hint="approvalOpinionHint"
+      :opinion-required="requiresApprovalOpinion"
       @close="closeApproveConfirmDialog"
       @confirm="confirmApproveRequest"
     />
@@ -784,6 +778,7 @@
       :title="`确认退回 ${request.id} 吗？`"
       :description="returnDialogDescription"
       :busy="returnBusy"
+      :application="isApplicationDocument"
       @close="closeReturnDialog"
       @confirm="confirmReturnRequest"
     />