修复公告模块的TypeScript错误和API调用

- 使用 apiClient 替换不存在的 apiRequest
- 修复可能为 undefined 的对象访问
- 添加可选链操作符和默认值

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/features/announcement/views/AnnouncementDetailView.vue

@@ -68,11 +68,11 @@ function getCategoryLabel(category: string) {
           size="small"
           class="category-tag"
           :style="{
-            borderColor: getCategoryInfo(announcement.category).color,
-            color: getCategoryInfo(announcement.category).color
+            borderColor: getCategoryInfo(announcement.category)?.color || '#94a3b8',
+            color: getCategoryInfo(announcement.category)?.color || '#94a3b8'
           }"
         >
-          <el-icon><component :is="getCategoryInfo(announcement.category).icon" /></el-icon>
+          <el-icon><component :is="getCategoryInfo(announcement.category)?.icon || Bell" /></el-icon>
           {{ getCategoryLabel(announcement.category) }}
         </el-tag>
         <el-tag v-if="announcement.is_pinned" type="warning" effect="plain" size="small">置顶</el-tag>