Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/core-js-compat/external.json

@@ -0,0 +1,21 @@
+{
+  "modules": {
+    "android": "61",
+    "bun": "0.1.1",
+    "chrome": "61",
+    "chrome-android": "61",
+    "deno": "1.0",
+    "edge": "16",
+    "firefox": "60",
+    "firefox-android": "60",
+    "ios": "10.3",
+    "node": "13.2",
+    "oculus": "4.0",
+    "opera": "48",
+    "opera-android": "45",
+    "opera_mobile": "45",
+    "quest": "4.0",
+    "safari": "10.1",
+    "samsung": "8.0"
+  }
+}