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/scripts/TravelReimbursementCreateView.js

@@ -176,6 +176,7 @@ import {
   buildWelcomeInsight,
   createMessage,
   filterAssistantSessionModes,
+  hasMeaningfulSessionMessages,
   resolveAssistantSessionMode,
   resolveKnowledgeRankLabel,
   resolveKnowledgeRankTone,
@@ -718,7 +719,7 @@ export default {
       return labels[currentInsight.value.intent] ?? 'AI 处理中'
     })
     const canDeleteCurrentSession = computed(
-      () => Boolean(conversationId.value) || messages.value.some((item) => item.role === 'user')
+      () => Boolean(conversationId.value) || hasMeaningfulSessionMessages(messages.value)
     )
     const latestReviewMessage = computed(() =>
       [...messages.value].reverse().find((item) =>
@@ -1029,6 +1030,7 @@ export default {
       handleGuidedShortcut,
       handleGuidedComposerSubmit,
       handleGuidedSuggestedAction,
+      handleSceneSelectionApplicationGate,
       resetGuidedFlowState
     } = useTravelReimbursementGuidedFlow({
       guidedFlowState,
@@ -1470,6 +1472,7 @@ export default {
       if (message?.suggestedActionsLocked) return
       if (applySuggestedActionPrefill(action)) return
       if (await handleGuidedSuggestedAction(message, action)) return
+      if (await handleSceneSelectionApplicationGate(message, action)) return
 
       if (actionType === ASSISTANT_SCOPE_ACTION_SWITCH) {
         const actionPayload = action?.payload && typeof action.payload === 'object' ? action.payload : {}