fix: Fetch error shown as string (not [object Object]); extract API error from response body; guard fetch availability

package.json

@@ -1,6 +1,6 @@
 {
   "name": "figma-ai-plugin",
-  "version": "0.1.5",
+  "version": "0.1.6",
   "private": true,
   "dependencies": {
     "@create-figma-plugin/ui": "^4.0.3",