fix: align canvas loading placeholder with start marker

Only show the image loading placeholder after the generation-start marker and avoid overlapping with the thinking state.

components/canvas/CanvasChatPanel.tsx

@@ -1071,7 +1071,7 @@ export function CanvasChatPanel({ chatId, initialMessages = [], onClose, onOpenF
     )
   );
   const currentModel = allModels.find(m => m.id === selectedModelId);
-  const shouldShowImageLoading = (isLoading || hasGeneratingMarker) && !!currentModel;
+  const shouldShowImageLoading = hasGeneratingMarker && !!currentModel;
   const loadingImageCount = imageParams.imageCount || 1;
 
   return (
@@ -1353,7 +1353,7 @@ export function CanvasChatPanel({ chatId, initialMessages = [], onClose, onOpenF
             />
           )}
           
-          {isLoading && (
+          {isLoading && !shouldShowImageLoading && (
             <div className="flex gap-3">
               <Avatar className="h-8 w-8 shrink-0 border shadow-sm">
                 <AvatarFallback className="bg-muted text-foreground">