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/ui/styles.css

@@ -2616,9 +2616,10 @@ button:disabled {
 }
 
 .layer-row {
+  position: relative;
   height: 46px;
   display: grid;
-  grid-template-columns: 32px minmax(0, 1fr) 26px 26px;
+  grid-template-columns: 18px 30px minmax(0, 1fr) 24px 24px;
   align-items: center;
   gap: 7px;
   padding: 4px 6px;
@@ -2627,6 +2628,18 @@ button:disabled {
   cursor: default;
 }
 
+.layer-row.is-dragging {
+  opacity: 0.42;
+}
+
+.layer-row.drop-before {
+  box-shadow: inset 0 2px 0 #2f80ff;
+}
+
+.layer-row.drop-after {
+  box-shadow: inset 0 -2px 0 #2f80ff;
+}
+
 .layer-row:hover,
 .layer-row.selected {
   background: #edf0f4;
@@ -2638,12 +2651,8 @@ button:disabled {
   font-weight: 760;
 }
 
-.layer-row.boolean-layer-group-row {
-  grid-template-columns: 14px 32px minmax(0, 1fr) 26px 26px;
-}
-
 .layer-row.nested {
-  grid-template-columns: 20px 28px minmax(0, 1fr) 24px 24px;
+  grid-template-columns: 20px 18px 28px minmax(0, 1fr) 24px 24px;
   padding-left: 12px;
 }
 
@@ -2654,6 +2663,38 @@ button:disabled {
   content: "";
 }
 
+.layer-disclosure,
+.layer-drag-handle {
+  width: 18px;
+  height: 26px;
+  display: grid;
+  place-items: center;
+  border: 0;
+  color: #8a92a0;
+  background: transparent;
+}
+
+.layer-disclosure {
+  border-radius: 6px;
+}
+
+.layer-disclosure:not(:disabled):hover {
+  color: #20242b;
+  background: #e4e8ee;
+}
+
+.layer-disclosure:disabled {
+  opacity: 0.32;
+}
+
+.layer-drag-handle {
+  opacity: 0.42;
+}
+
+.layer-row:hover .layer-drag-handle {
+  opacity: 0.8;
+}
+
 .layer-row.is-hidden {
   opacity: 0.58;
 }