Pass upstream policy errors through PyPI proxy to users

- Add _parse_upstream_error() to extract policy messages from JFrog/Artifactory
- Pass through 403 and other 4xx errors with detailed messages
- Pin babel and electron-to-chromium to older versions for CI compatibility

frontend/package.json

@@ -38,6 +38,14 @@
     "rollup": "4.52.4",
     "caniuse-lite": "1.0.30001692",
     "baseline-browser-mapping": "2.9.5",
-    "lodash": "4.17.21"
+    "lodash": "4.17.21",
+    "electron-to-chromium": "1.5.72",
+    "@babel/core": "7.26.0",
+    "@babel/traverse": "7.26.4",
+    "@babel/types": "7.26.3",
+    "@babel/compat-data": "7.26.3",
+    "@babel/parser": "7.26.3",
+    "@babel/generator": "7.26.3",
+    "@babel/code-frame": "7.26.2"
   }
 }