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/go.sum

@@ -34,8 +34,6 @@ github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
 github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
-github.com/gopherjs/gopherjs v1.21.0 h1:5HEGrz+XhpCchubMGzuyLuGoCTlL/yCT7sGsT5Se/dw=
-github.com/gopherjs/gopherjs v1.21.0/go.mod h1:R2HIOen3IzYSzvmvkeD8WOfiLN9wueR/T5Y+6z326Ck=
 github.com/gorilla/websocket v1.5.3 h1:saDtZ6Pbx/0u+bgYQ3q96pZgCzfhKXGPqt7kZ72aNNg=
 github.com/gorilla/websocket v1.5.3/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
 github.com/grafana/pyroscope-go v1.3.0 h1:t3Jehad8vvqN4oRAB0LdmfQ5ZSUXQw3asoft+K4GAT8=
@@ -81,8 +79,6 @@ github.com/minio/minio-go/v7 v7.2.1 h1:PfBfwvKB/MmqyN8Vb1G9voWisaM9OrLv+WwOvMwS9
 github.com/minio/minio-go/v7 v7.2.1/go.mod h1:EU9hENAStx/xXduNdrGO5e4X5vk19NtgB+RIPjZO8o0=
 github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq1c1nUAm88MOHcQC9l5mIlSMApZMrHA=
 github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822/go.mod h1:+n7T8mK8HuQTcFwEeznm/DIxMOiR9yIdICNftLE1DvQ=
-github.com/oov/psd v0.0.0-20260122084234-c463b6a89e2f h1:67UmABhaufAdSFaRO/wcaTn0XgHWCirjwfFf21ZNPAc=
-github.com/oov/psd v0.0.0-20260122084234-c463b6a89e2f/go.mod h1:GHI1bnmAcbp96z6LNfBJvtrjxhaXGkbsk967utPlvL8=
 github.com/openzipkin/zipkin-go v0.4.3 h1:9EGwpqkgnwdEIJ+Od7QVSEIH+ocmm5nPat0G7sjsSdg=
 github.com/openzipkin/zipkin-go v0.4.3/go.mod h1:M9wCJZFWCo2RiY+o1eBCEMe0Dp2S5LDHcMZmk3RmK7c=
 github.com/pelletier/go-toml/v2 v2.3.1 h1:MYEvvGnQjeNkRF1qUuGolNtNExTDwct51yp7olPtrEc=