feat(canvas): frame-based layer separation UI

Render and manage the separated layers produced by the edit-elements
action as a manual-frame container with attached image and text
children.

- canvasNodeOps: frame container/child helpers, render ordering,
  auto-attach nodes to containing frames, and layer reordering
- CanvasWorkspace wires frame-aware selection, drag, resize, hide/lock,
  and layer reorder through the new ops
- CanvasPanels: expandable layer tree with rename/reorder
- designGateway.submitGeneratorTask + GeneratorTaskSubmitResponse and
  bbox/generator_name artifact metadata in the domain
- ArtboardPreview shows a working state for generating frame layers
- i18n: expandLayer / renameLayer / layerTitlePlaceholder; styles

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/domain/design.ts

@@ -148,7 +148,9 @@ export type GeneratorTaskArtifact = {
   content: string;
   metadata: {
     artifact_id?: string;
+    bbox?: number[];
     format?: string;
+    generator_name?: string;
     height?: number;
     label?: string;
     node_id?: string;
@@ -177,6 +179,14 @@ export type GeneratorTaskResponse = {
   };
 };
 
+export type GeneratorTaskSubmitResponse = {
+  code: number;
+  message: string;
+  data: {
+    generator_task_id: string;
+  };
+};
+
 export type DeleteProjectResponse = {
   id: string;
   deleted: boolean;