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/components/CanvasNodeContextMenu/index.tsx

@@ -105,7 +105,7 @@ export function CanvasNodeContextMenu({
           <ChevronDown className="context-menu-chevron" size={14} />
         </ContextMenuSubTrigger>
         <ContextMenuSubContent className="ui-context-menu-content image-export-submenu">
-          {handlers.exportFormats.map((format) => (
+          {handlers.exportFormats.filter((format) => format !== "psd").map((format) => (
             <ContextMenuItem key={format} onSelect={() => handlers.exportAs(format)}>
               <span>{format.toUpperCase()}</span>
             </ContextMenuItem>