fix: add GLSLShader to canvas image preview node types (#9198)

## Summary

Add `GLSLShader` to `CANVAS_IMAGE_PREVIEW_NODE_TYPES` so GLSL shader
previews are promoted through subgraph nodes.

## Changes

- Add `'GLSLShader'` to the `CANVAS_IMAGE_PREVIEW_NODE_TYPES` set in
`src/composables/node/useNodeCanvasImagePreview.ts`

## Context

GLSLShader node previews were not showing on parent subgraph nodes
because `CANVAS_IMAGE_PREVIEW_NODE_TYPES` only included `PreviewImage`
and `SaveImage`. The `$$canvas-image-preview` pseudo-widget was never
created for GLSLShader nodes, so the promotion system had nothing to
promote. This degraded the UX of all 12 shipped GLSL blueprint subgraphs
— users couldn't see shader output previews without expanding the
subgraph.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9198-fix-add-GLSLShader-to-canvas-image-preview-node-types-3126d73d3650817dbe9beab4bdeaa414)
by [Unito](https://www.unito.io)

src/composables/node/useNodeCanvasImagePreview.ts

@@ -2,7 +2,11 @@ import type { LGraphNode } from '@/lib/litegraph/src/litegraph'
 import { useImagePreviewWidget } from '@/renderer/extensions/vueNodes/widgets/composables/useImagePreviewWidget'
 
 export const CANVAS_IMAGE_PREVIEW_WIDGET = '$$canvas-image-preview'
-const CANVAS_IMAGE_PREVIEW_NODE_TYPES = new Set(['PreviewImage', 'SaveImage'])
+const CANVAS_IMAGE_PREVIEW_NODE_TYPES = new Set([
+  'PreviewImage',
+  'SaveImage',
+  'GLSLShader'
+])
 
 export function supportsVirtualCanvasImagePreview(node: LGraphNode): boolean {
   return CANVAS_IMAGE_PREVIEW_NODE_TYPES.has(node.type)