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/package.json

@@ -0,0 +1,20 @@
+{
+  "name": "is-module",
+  "description": "check if a source string is an es6 module",
+  "keywords": ["es6", "module", "modules"],
+  "version": "1.0.0",
+  "author": {
+    "name": "Jonathan Ong",
+    "email": "me@jongleberry.com",
+    "url": "http://jongleberry.com",
+    "twitter": "https://twitter.com/jongleberry"
+  },
+  "license": "MIT",
+  "repository": "component/is-module",
+  "devDependencies": {
+    "mocha": "*"
+  },
+  "scripts": {
+    "test": "mocha --reporter spec --bail"
+  }
+}