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/package.json

@@ -17,6 +17,7 @@
     "@radix-ui/react-scroll-area": "^1.2.13",
     "@radix-ui/react-switch": "^1.3.2",
     "@radix-ui/react-tabs": "^1.1.16",
+    "ag-psd": "31.0.2",
     "lucide-react": "^1.23.0",
     "next": "^16.2.10",
     "react": "^19.2.7",