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/get-own-enumerable-property-symbols/README.md

@@ -0,0 +1,15 @@
+# get-own-enumerable-property-symbols [![Build Status](https://travis-ci.org/mightyiam/get-own-enumerable-property-symbols.svg?branch=master)](https://travis-ci.org/mightyiam/get-own-enumerable-property-symbols)
+
+Returns an array of all *enumerable* symbol properties found directly upon a given object.
+
+Similar to [`Object.getOwnPropertySymbols`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertySymbols)
+but only [enumerable](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties) keys.
+
+```js
+import getOwnEnumPropSymbols from 'get-own-enumerable-property-symbols'
+
+getOwnEnumPropSymbols({ [Symbol()]: undefined })
+// [Symbol()]
+getOwnEnumPropSymbols(Object.defineProperty({}, Symbol(), {enumerable: false}))
+// []
+```