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/i18n/locales/en-US.json

@@ -66,6 +66,7 @@
   "recentProjects": "Recent Projects",
   "viewAll": "View all",
   "collapse": "Collapse",
+  "expandLayer": "Expand layer",
   "newProject": "New Project",
   "continueEditing": "Continue",
   "inspiration": "Inspiration",
@@ -126,6 +127,8 @@
   "renameCanvasTitle": "Rename canvas",
   "renameImageNode": "Rename image",
   "imageTitlePlaceholder": "Image name",
+  "renameLayer": "Rename layer",
+  "layerTitlePlaceholder": "Layer name",
   "projectTitlePlaceholder": "Name this canvas",
   "select": "Select",
   "pan": "Move",

frontend/src/i18n/locales/zh-CN.json

@@ -66,6 +66,7 @@
   "recentProjects": "最近项目",
   "viewAll": "查看全部",
   "collapse": "收起",
+  "expandLayer": "展开图层",
   "newProject": "新建项目",
   "continueEditing": "继续编辑",
   "inspiration": "灵感发现",
@@ -126,6 +127,8 @@
   "renameCanvasTitle": "重命名画布",
   "renameImageNode": "重命名图片",
   "imageTitlePlaceholder": "输入图片名称",
+  "renameLayer": "重命名图层",
+  "layerTitlePlaceholder": "输入图层名称",
   "projectTitlePlaceholder": "输入画布名称",
   "select": "选择",
   "pan": "移动",