fix(aiot): ROI 显示修复 + 前端截图策略优化

- aiRoi.js: getSnapUrl 改为直接返回 /snap/image 代理 URL，
  非 force 模式不再触发 Edge 截图（从 DB 读持久化截图）
- roiConfig.vue: fetchSnap 适配新 API（直接使用返回的 URL）
- RoiCanvas.vue:
  · 添加 ResizeObserver 确保容器尺寸变化时重新初始化 canvas
  · onImageError 兜底初始化 canvas（截图失败仍可绘制/查看 ROI）
  · snapUrl watcher 触发 canvas 重初始化

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

web/src/views/roiConfig/components/RoiCanvas.vue

@@ -43,7 +43,8 @@ export default {
       currentPoint: null,
       polygonPoints: [],
       loading: true,
-      errorMsg: ''
+      errorMsg: '',
+      resizeObserver: null
     }
   },
   watch: {
@@ -57,16 +58,30 @@ export default {
     snapUrl() {
       this.loading = true
       this.errorMsg = ''
+      this.$nextTick(() => this.initCanvas())
     }
   },
   mounted() {
     this.$nextTick(() => {
       this.initCanvas()
+      // ResizeObserver 确保容器尺寸变化时重新初始化 canvas
+      if (this.$refs.wrapper && typeof ResizeObserver !== 'undefined') {
+        this.resizeObserver = new ResizeObserver(() => {
+          if (this.$refs.wrapper && this.$refs.wrapper.clientWidth > 0) {
+            this.initCanvas()
+          }
+        })
+        this.resizeObserver.observe(this.$refs.wrapper)
+      }
       window.addEventListener('resize', this.handleResize)
     })
   },
   beforeDestroy() {
     window.removeEventListener('resize', this.handleResize)
+    if (this.resizeObserver) {
+      this.resizeObserver.disconnect()
+      this.resizeObserver = null
+    }
   },
   methods: {
     onImageLoad() {
@@ -78,6 +93,8 @@ export default {
     onImageError() {
       this.loading = false
       this.errorMsg = '截图加载失败，请确认摄像头正在拉流'
+      // 关键：截图失败也初始化 canvas，使 ROI 区域可见可操作
+      this.$nextTick(() => this.initCanvas())
     },
     initCanvas() {
       const canvas = this.$refs.canvas