fix(aiot): 告警截图 Image 组件添加 fallback 兜底显示

列表缩略图和详情弹窗的 Image 组件均添加 fallback SVG，
当图片 URL 无效或加载失败时显示友好占位符而非损坏图标。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web-antd/src/views/aiot/alarm/list/index.vue

@@ -252,6 +252,7 @@ const [Grid, gridApi] = useVbenVxeGrid({
           :width="40"
           :height="40"
           :preview="{ src: row.ossUrl || row.snapshotUrl }"
+          :fallback="`data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='40' height='40'><rect width='40' height='40' fill='%23f0f0f0'/><text x='50%25' y='55%25' dominant-baseline='middle' text-anchor='middle' fill='%23bbb' font-size='12'>-</text></svg>`"
           style="object-fit: cover; border-radius: 4px; cursor: pointer"
         />
         <span v-else class="text-gray-400">-</span>
@@ -356,6 +357,7 @@ const [Grid, gridApi] = useVbenVxeGrid({
               :src="currentAlert.ossUrl || currentAlert.snapshotUrl"
               :width="300"
               :preview="{ src: currentAlert.ossUrl || currentAlert.snapshotUrl }"
+              :fallback="`data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='300' height='200'><rect width='300' height='200' fill='%23f5f5f5'/><text x='50%25' y='50%25' dominant-baseline='middle' text-anchor='middle' fill='%23bbb' font-size='14'>图片加载失败</text></svg>`"
             />
           </div>
           <div class="mt-1">