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/is-module/component.json

@@ -0,0 +1,11 @@
+{
+  "name": "is-module",
+  "description": "check if a source string is an es6 module",
+  "keywords": ["es6", "module", "modules"],
+  "version": "1.0.0",
+  "license": "MIT",
+  "repo": "component/is-module",
+  "scripts": [
+    "index.js"
+  ]
+}