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>

server/internal/domain/design/agent.go

@@ -74,25 +74,6 @@ type GeneratorTaskInputArgs struct {
 	DstBox       *NormalizedBox
 }
 
-type LayeredDocumentRequest struct {
-	ImageURL       string
-	Prompt         string
-	Model          string
-	Size           string
-	OutputFormat   string
-	IdempotencyKey string
-}
-
-type LayeredDocument struct {
-	Content     string
-	ContentType string
-	Data        []byte
-}
-
-type LayeredDocumentGenerator interface {
-	GenerateLayeredDocument(ctx context.Context, req LayeredDocumentRequest) (LayeredDocument, error)
-}
-
 type GeneratorTaskSubmitRequest struct {
 	CID           string
 	ProjectID     string