Fix copying from multi-outputs with clipspace (#6210)

`copyToClipspace` sets the `paintedIndex` and `combinedIndex` regardless
of if any editing was actually performed on the image. Previously, they
were set to be simple increments above the currently selected image.

When attempting to copy from a node previewing multiple images, these
indexes may refer to images that already exist.
This is resolved by instead setting the indexes to be used for
paintedIndex and combinedIndex to always be beyond the range of current
images.
<img width="998" height="761" alt="image"
src="https://github.com/user-attachments/assets/dacd46b3-9029-45e3-a6b3-07e971125a14"
/>


Resolves #6202

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6210-Fix-copying-from-multi-outputs-with-clipspace-2956d73d365081c5a07be4ddb0b352a3)
by [Unito](https://www.unito.io)

src/scripts/app.ts

@@ -379,8 +379,8 @@ export class ComfyApp {
       selectedIndex = node.imageIndex
     }
 
-    const paintedIndex = selectedIndex + 1
-    const combinedIndex = selectedIndex + 2
+    const paintedIndex = imgs ? imgs.length + 1 : 1
+    const combinedIndex = imgs ? imgs.length + 2 : 2
 
     // for vueNodes mode
     const images =