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/svc/service_context.go

@@ -59,7 +59,6 @@ func NewServiceContext(c config.Config) *ServiceContext {
 	textExtractor := newTextExtractor(c)
 	service := application.NewDesignService(repo, agentRunner, assets, researcher, textExtractor, creativeAgent)
 	service.SetBackgroundRemover(newBackgroundRemover(c))
-	service.SetLayeredDocumentGenerator(newLayeredDocumentGenerator(c))
 	service.SetObjectRecognizer(newObjectRecognizer(c))
 	if mockupAnalyzer, ok := creativeAgent.(design.MockupModelAnalyzer); ok {
 		service.SetMockupModelAnalyzer(mockupAnalyzer)
@@ -318,22 +317,6 @@ func newImageGenerator(c config.Config) piagent.ImageGenerator {
 	return client
 }
 
-func newLayeredDocumentGenerator(c config.Config) design.LayeredDocumentGenerator {
-	opts := c.Agent.Image
-	apiKey := resolveImageAPIKey(opts)
-	client := piagent.NewImageClient(piagent.ImageClientOptions{
-		BaseURL:                      opts.BaseURL,
-		APIKey:                       apiKey,
-		InputImageTransport:          opts.InputImageTransport,
-		TimeoutSeconds:               opts.TimeoutSeconds,
-		ResponseHeaderTimeoutSeconds: opts.ResponseHeaderTimeoutSeconds,
-	})
-	if client == nil {
-		return nil
-	}
-	return client
-}
-
 func newTextExtractor(c config.Config) design.TextExtractor {
 	opts := c.Agent.Text
 	switch opts.Driver {