feat(canvas): build PSD export in the browser from canvas layers

Move PSD generation off the server. The image-generation API only
produces raster formats, so drop the PSD request path, the oov/psd
decoder, and the LayeredDocumentGenerator wiring. Layer separation now
returns clean background, foreground, and text_render_data artifacts
into a transparent frame.

Export that frame (or any image node) as PSD from the canvas context
menu: ag-psd assembles layers, rotation/flip, opacity, editable text,
and stroke effects from the current canvas state at download time.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/pages/CanvasWorkspace/index.tsx

@@ -2270,8 +2270,8 @@ export function CanvasWorkspace({ projectId }: { projectId: string }) {
     toggleLocked: () => toggleLayerLocked(node.id),
     flipHorizontal: () => toggleNodeTransform(node.id, "flipX"),
     flipVertical: () => toggleNodeTransform(node.id, "flipY"),
-    exportFormats: node.type === "text" ? ["svg"] : ["png", "jpg", "svg"],
-    exportAs: (format) => exportCanvasNode(node, format),
+    exportFormats: node.type === "text" ? ["svg"] : ["png", "jpg", "svg", "psd"],
+    exportAs: (format) => exportCanvasNode(node, format, { nodes: nodesRef.current, hiddenNodeIds }),
     deleteNode: () => removeNode(node.id)
   });