优化添加背景页面功能逻辑，修复连续上传多张图片导致的换颜色无效的问题

src/views/tools/add_background/add_background.vue

@@ -2218,7 +2218,7 @@ const initializeCanvasWithBackground = async (bgColor: string) => {
 
 // Initialize canvas from saved result (for history items)
 const initializeCanvasFromResult = async (item: HistoryItem) => {
-  if (!canvasRef.value || !item.resultImage) {
+  if (!canvasRef.value || !item.originalImageUrl) {
     processing.value = false
     return
   }
@@ -2237,24 +2237,24 @@ const initializeCanvasFromResult = async (item: HistoryItem) => {
       selection: false
     })
     
-    // Load the result image directly
-    const resultImg = await FabricImage.fromURL(item.resultImage, {
+    // Load the ORIGINAL transparent image (not the result)
+    const img = await FabricImage.fromURL(item.originalImageUrl, {
       crossOrigin: 'anonymous'
     })
     
-    if (!resultImg || !fabricCanvas) {
+    if (!img || !fabricCanvas) {
       processing.value = false
       return
     }
     
     // Get image natural dimensions
-    const imgWidth = resultImg.width || 1
-    const imgHeight = resultImg.height || 1
+    const imgWidth = img.width || 1
+    const imgHeight = img.height || 1
     
     // Set canvas to actual image dimensions
     fabricCanvas.setDimensions({ width: imgWidth, height: imgHeight })
     
-    resultImg.set({
+    img.set({
       left: 0,
       top: 0,
       selectable: false,
@@ -2263,8 +2263,15 @@ const initializeCanvasFromResult = async (item: HistoryItem) => {
       scaleY: 1
     })
     
-    fabricCanvas.add(resultImg)
-    fabricCanvas.centerObject(resultImg)
+    // Apply saved background color or leave transparent
+    if (item.backgroundColor) {
+      fabricCanvas.backgroundColor = item.backgroundColor
+    } else {
+      fabricCanvas.backgroundColor = null
+    }
+    
+    fabricCanvas.add(img)
+    fabricCanvas.centerObject(img)
     fabricCanvas.renderAll()
     
     // Adjust visual display size to fit container