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/core/graph/subgraph/promotionUtils.ts

@@ -227,6 +227,29 @@ export function promoteRecommendedWidgets(subgraphNode: SubgraphNode) {
     // defer. Core $$ preview widgets are the lazy path that needs updatePreviews.
     if (hasPreviewWidget()) continue
 
+    // Nodes in CANVAS_IMAGE_PREVIEW_NODE_TYPES support a virtual $$
+    // preview widget. Eagerly promote it so getPseudoWidgetPreviewTargets
+    // includes this node and onDrawBackground can call updatePreviews on it
+    // once execution outputs arrive.
+    if (supportsVirtualCanvasImagePreview(node)) {
+      if (
+        !store.isPromoted(
+          subgraphNode.rootGraph.id,
+          subgraphNode.id,
+          String(node.id),
+          CANVAS_IMAGE_PREVIEW_WIDGET
+        )
+      ) {
+        store.promote(
+          subgraphNode.rootGraph.id,
+          subgraphNode.id,
+          String(node.id),
+          CANVAS_IMAGE_PREVIEW_WIDGET
+        )
+      }
+      continue
+    }
+
     // Also schedule a deferred check: core $$ widgets are created lazily by
     // updatePreviews when node outputs are first loaded.
     requestAnimationFrame(() => updatePreviews(node, promotePreviewWidget))