优化添加背景页面切换背景图的逻辑，修复抖动问题

src/views/tools/add_background/add_background.vue

@@ -829,19 +829,14 @@ const applyBackgroundImage = async (imageUrl: string) => {
   if (!fabricCanvas || !uploadedImage.value || !uploadedImageUrl.value) return
   
   try {
-    processing.value = true
+    // Don't set processing.value = true to avoid overlay flicker
     
-    // Save the original foreground image reference before clearing
-    const currentObjects = fabricCanvas.getObjects()
-    const foregroundImg = currentObjects.length > 0 ? currentObjects[0] : null
-    
-    // Load background image
+    // Load background image first (canvas not changed yet, user sees nothing)
     const bgImg = await FabricImage.fromURL(imageUrl, {
       crossOrigin: 'anonymous'
     })
     
     if (!bgImg || !fabricCanvas) {
-      processing.value = false
       return
     }
     
@@ -866,42 +861,28 @@ const applyBackgroundImage = async (imageUrl: string) => {
       evented: false
     })
     
-    // Clear canvas
-    fabricCanvas.clear()
+    // Get current objects
+    const currentObjects = fabricCanvas.getObjects()
     
-    // Add background image first (at the bottom)
+    // Disable auto-rendering to prevent flicker
+    fabricCanvas.renderOnAddRemove = false
+    
+    // Remove old background images (keep only the last object - foreground)
+    // This keeps the foreground visible at all times
+    if (currentObjects.length > 1) {
+      const objectsToRemove = currentObjects.slice(0, -1)
+      objectsToRemove.forEach(obj => fabricCanvas.remove(obj))
+    }
+    
+    // Add new background image at the bottom
     fabricCanvas.add(bgImg)
+    fabricCanvas.sendObjectToBack(bgImg)
     
-    // Re-load and add the foreground image (original uploaded image)
-    if (foregroundImg) {
-      // Clone the foreground image to avoid issues
-      const newForegroundImg = await FabricImage.fromURL(uploadedImageUrl.value, {
-        crossOrigin: 'anonymous'
-      })
-      
-      if (newForegroundImg) {
-        newForegroundImg.set({
-          left: canvasWidth / 2,
-          top: canvasHeight / 2,
-          originX: 'center',
-          originY: 'center',
-          selectable: false,
-          evented: false
-        })
-        
-        fabricCanvas.add(newForegroundImg)
-      }
-    }
-    
-    // Ensure correct layer order: background at bottom, foreground on top
-    const allObjects = fabricCanvas.getObjects()
-    if (allObjects.length === 2) {
-      // Move background (first object) to back
-      fabricCanvas.sendObjectToBack(allObjects[0])
-      // Move foreground (second object) to front
-      fabricCanvas.bringObjectToFront(allObjects[1])
-    }
+    // Clear background color since we're using background image
+    fabricCanvas.backgroundColor = null
     
+    // Re-enable auto-rendering and render once
+    fabricCanvas.renderOnAddRemove = true
     fabricCanvas.renderAll()
     
     // Export result
@@ -930,8 +911,6 @@ const applyBackgroundImage = async (imageUrl: string) => {
       historyList.value[currentHistoryIndex.value].resultImage = dataUrl
     }
     
-    processing.value = false
-    
     // Auto-close mobile color picker on mobile
     if (window.innerWidth <= 768) {
       closeMobileColorPicker()
@@ -939,7 +918,6 @@ const applyBackgroundImage = async (imageUrl: string) => {
   } catch (error: any) {
     console.error('Apply background image error:', error)
     message.error(t('Failed to apply background image'))
-    processing.value = false
   }
 }