fix: CanvasPreview crash - transform DBElement properties_json to CADElement properties

Backend returns DBElement with properties_json (string), not properties (object).
CanvasPreview accessed el.properties.stroke directly → undefined → React crash → white screen.
Fix: transform raw API response to CADElement format with JSON.parse(properties_json).

frontend/src/pages/Dashboard.tsx

@@ -33,8 +33,19 @@ function CanvasPreview({ project, token }: { project: Project; token: string })
       try {
         const drawings = await getDrawings(token, project.id);
         if (drawings.length === 0) { if (!cancelled) setElements([]); return; }
-        const els = await getElements(token, drawings[0].id);
+        const rawEls = await getElements(token, drawings[0].id);
-        if (!cancelled) setElements(els);
+        // API may return DBElement with properties_json (string) instead of properties (object)
+        const transformed: CADElement[] = rawEls.map((el: any) => ({
+          id: el.id,
+          type: el.type,
+          layerId: el.layer_id ?? el.layerId ?? '',
+          x: el.x ?? 0,
+          y: el.y ?? 0,
+          width: el.width ?? 0,
+          height: el.height ?? 0,
+          properties: typeof el.properties_json === 'string' ? JSON.parse(el.properties_json || '{}') : (el.properties ?? {}),
+        }));
+        if (!cancelled) setElements(transformed);
       } catch {
         if (!cancelled) setElements([]);
       } finally {