refactor(top-nav-bar): add custom back prop

- add customBack prop to enable explicit custom back handling
- rewrite back navigation logic to use the new prop instead of inferring from emit listeners
- remove unused getCurrentInstance import
- update three AIGC page components to use the custom-back prop

src/pages-aigc/pointsDetails/pointsDetails.vue

@@ -3,7 +3,7 @@
     safe-area-inset-bottom @query="queryList">
     <template #top>
       <TopNavBar title="积分明细" background="transparent" title-color="#172033" back-icon-color="#172033"
-        :align-with-menu-button="true" :z-index="3" @back="handleBack" />
+        custom-back :align-with-menu-button="true" :z-index="3" @back="handleBack" />
     </template>
 
     <view class="points-details-content">

src/pages-aigc/record/record.vue

@@ -4,7 +4,7 @@
     <template #top>
       <view class="aigc-record-nav">
         <TopNavBar title="最近任务" background="transparent" title-color="#162235" back-icon-color="#162235"
-          :align-with-menu-button="true" :z-index="1" @back="handleBack" />
+          custom-back :align-with-menu-button="true" :z-index="1" @back="handleBack" />
       </view>
     </template>
 

src/pages-aigc/sharedWork/sharedWork.vue

@@ -2,7 +2,7 @@
   <view class="shared-work-page">
     <view class="shared-work-nav">
       <TopNavBar :title="shareDetail.templateName || '旅行作品'" background="#fff" title-color="#173a5f"
-        back-icon-color="#173a5f" :align-with-menu-button="true" :z-index="3" @back="handleBack" />
+        back-icon-color="#173a5f" custom-back :align-with-menu-button="true" :z-index="3" @back="handleBack" />
     </view>
 
     <view class="shared-work-scroll">