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/ui/lib/cadImport.worker.ts

@@ -0,0 +1,30 @@
+import { convertCadBufferToSvg, type CadFormat } from "./cadConversion";
+
+type CadWorkerRequest = {
+  buffer: ArrayBuffer;
+  format: CadFormat;
+};
+
+type CadWorkerResponse =
+  | { ok: true; svg: string }
+  | { ok: false; error: string };
+
+type WorkerScope = {
+  onmessage: ((event: MessageEvent<CadWorkerRequest>) => void) | null;
+  postMessage: (message: CadWorkerResponse) => void;
+};
+
+const workerScope = globalThis as unknown as WorkerScope;
+
+workerScope.onmessage = (event) => {
+  try {
+    const { buffer, format } = event.data;
+    const svg = convertCadBufferToSvg(buffer, format);
+    workerScope.postMessage({ ok: true, svg });
+  } catch (error) {
+    workerScope.postMessage({
+      ok: false,
+      error: error instanceof Error ? error.message : String(error)
+    });
+  }
+};