feat(canvas): per-generator target size, placement and draft preservation

Persist each image-generator node's target resolution in metadata and
surface it in the selection frame and node preview. Place new generators
without overlapping existing ones, keep local draft generators when a
compressed snapshot omits them, and add a dedicated selection-frame
variant plus scale-aware preview styling.

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

frontend/src/ui/styles.css

@@ -2365,6 +2365,14 @@ button:disabled {
   right: calc(10px / var(--selection-scale));
 }
 
+.image-generator-selection-frame {
+  border-color: transparent;
+}
+
+.image-generator-selection-frame .moteva-selection-meta {
+  display: none;
+}
+
 .pen-stroke-selection-frame .moteva-selection-meta {
   display: none;
 }