fix(canvas): label the generating skeleton with status, not the planned title

Show a localized "Generating" label on the artboard generation skeleton
while keeping the planned title as the aria-label, so in-progress nodes read
as a status instead of prematurely displaying their eventual name.

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

frontend/src/i18n/locales/en-US.json

@@ -443,6 +443,7 @@
   "cadImportFailed": "CAD conversion failed. Check that the file is complete and uses a supported version.",
   "uploadingImage": "Uploading",
   "extractingText": "Extract text",
+  "generatingImage": "Generating",
   "textNode": "Text node",
   "frameNode": "Frame",
   "generateImage": "Generate image",