pagedraw: composite draw layer into reference image for default generation case

3D nodes and drawings on the canvas were missing from the reference image
sent to the model in the default generation path. The fix mirrors the Nano
Banana Pro approach: render a composite of the base image and the draw layer,
upload it via uploadSceneSnapshot, and pass it as scene_image_media_token
rather than putting editedImageToken in image_media_tokens.

frontend/apps/artcraft/app/src/pages/PageDraw/PageDraw.tsx

@@ -854,15 +854,33 @@ const PageDraw = () => {
           result = await EnqueueEditImage(request);
         } else {
           // CASE 3 - DEFAULT
+          // Tech debt: we upload the composite first then enqueue the generation as two
+          // separate HTTP requests. Ideally the backend API would accept inline image
+          // bytes in the generation request so this could be a single call.
+          const compositeFile = await getCompositeCanvasFile();
+
+          if (!compositeFile) {
+            console.error("Failed to create composite canvas");
+            return;
+          }
+
+          const api = new PromptsApi();
+          const snapshotResult = await api.uploadSceneSnapshot({
+            screenshot: compositeFile,
+          });
+
+          if (!snapshotResult.data) {
+            console.error("Failed to upload scene snapshot");
+            return;
+          }
+
           const imgs = options?.images || [];
           const request: EnqueueEditImageRequest = {
             model: selectedImageModel,
-            image_media_tokens: [
-              editedImageToken,
-              ...imgs
-                .filter((img) => img.mediaToken !== editedImageToken)
-                .map((img) => img.mediaToken),
-            ].filter((t) => t.length > 0),
+            scene_image_media_token: snapshotResult.data,
+            image_media_tokens: imgs
+              .map((img) => img.mediaToken)
+              .filter((t) => t.length > 0),
             disable_system_prompt: true,
             prompt: prompt,
             image_count: generationCount,
@@ -874,9 +892,6 @@ const PageDraw = () => {
           if (options?.selectedProvider) {
             request.provider = options.selectedProvider;
           }
-          // if (selectedImageModel?.supportsNewAspectRatio()) {
-          //   request.common_aspect_ratio = commonAspectRatio;
-          // }
           result = await EnqueueEditImage(request);
         }
         if (result?.status === "success") {