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/infrastructure/designGateway.ts

@@ -11,6 +11,7 @@ import type {
   DeleteProjectsResponse,
   ExtractNodeTextResponse,
   GeneratorTaskResponse,
+  GeneratorTaskSubmitResponse,
   GenerateResult,
   MergeLayersRequest,
   NodeActionRequest,
@@ -358,6 +359,18 @@ export const designGateway = {
     return request<GeneratorTaskResponse>(`/api/generator/tasks?${params.toString()}`);
   },
 
+  submitGeneratorTask(projectId: string, generatorName: string, inputArgs: Record<string, unknown>) {
+    return request<GeneratorTaskSubmitResponse>("/api/generator/tasks", {
+      method: "POST",
+      body: JSON.stringify({
+        cid: createClientId(),
+        project_id: projectId,
+        generator_name: generatorName,
+        input_args: inputArgs
+      })
+    });
+  },
+
   recognizeObject(imageUrl: string, boundingBox: NormalizedBox, lang = "zh") {
     return request<RecognizeObjectResponse>("/api/context/vision/recognize_object", {
       method: "POST",