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/application/generator_task_test.go

@@ -7,6 +7,15 @@ import (
 	"img_infinite_canvas/internal/domain/design"
 )
 
+func TestNormalizeLayerSeparationInputDoesNotClaimPSDOutput(t *testing.T) {
+	input := normalizeLayerSeparationInput(design.GeneratorTaskInputArgs{
+		ImageURL: "https://example.com/source.png",
+	})
+	if input.OutputFormat != "" {
+		t.Fatalf("expected layer separation task output format to describe no upstream raster format, got %q", input.OutputFormat)
+	}
+}
+
 func TestGeneratorTaskStatusWaitsForActiveCanvasWork(t *testing.T) {
 	threadID := "thread-image"
 	project := design.Project{