feat(canvas): import DWG/DXF CAD drawings as canvas assets

Accept DWG and DXF files (up to 24 MB) through the canvas asset picker and
drag-and-drop. Conversion runs locally in a browser worker via acad-ts,
uploads a sanitized derived SVG, and leaves the source file untouched. DXF
covers ASCII and binary drawings; DWG covers AutoCAD R14 through 2018.

Rename the image-only upload input/handler to a generic canvas asset flow,
add vector-image node detection, tag imported SVGs as vector-image nodes,
localize CAD import status/error messages, and switch the Next build to the
webpack compiler so the worker bundles correctly. Add a node --test suite
covering CAD conversion and vector image helpers.

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

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

@@ -432,7 +432,15 @@
   "canvasCursor": "Canvas status",
   "imageNode": "Image node",
   "uploadImageNode": "Upload image node",
+  "uploadCanvasAsset": "Upload image or CAD file",
   "imageUploadSuccess": "Image uploaded",
+  "cadImportConverting": "Converting CAD drawing",
+  "cadImportSuccess": "Imported {count} CAD drawing(s)",
+  "cadImportTooLarge": "CAD files must be 24 MB or smaller.",
+  "cadImportTimeout": "CAD conversion timed out. Simplify the drawing and try again.",
+  "cadImportEmpty": "The CAD drawing has no visible model-space content.",
+  "cadImportUnsupported": "This browser cannot convert the selected CAD file.",
+  "cadImportFailed": "CAD conversion failed. Check that the file is complete and uses a supported version.",
   "uploadingImage": "Uploading",
   "extractingText": "Extract text",
   "textNode": "Text node",