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/es-abstract/2022/AddToKeptObjects.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var SLOT = require('internal-slot');
+
+var $TypeError = require('es-errors/type');
+var isObject = require('es-object-atoms/isObject');
+
+var ClearKeptObjects = require('./ClearKeptObjects');
+
+// https://262.ecma-international.org/12.0/#sec-addtokeptobjects
+
+module.exports = function AddToKeptObjects(object) {
+	if (!isObject(object)) {
+		throw new $TypeError('Assertion failed: `object` must be an Object');
+	}
+	var arr = SLOT.get(ClearKeptObjects, '[[es-abstract internal: KeptAlive]]');
+	arr[arr.length] = object;
+};