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/2023/CanBeHeldWeakly.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var isObject = require('es-object-atoms/isObject');
+
+var KeyForSymbol = require('./KeyForSymbol');
+
+// https://262.ecma-international.org/14.0/#sec-canbeheldweakly
+
+module.exports = function CanBeHeldWeakly(v) {
+	if (isObject(v)) {
+		return true; // step 1
+	}
+	if (typeof v === 'symbol' && typeof KeyForSymbol(v) === 'undefined') {
+		return true; // step 2
+	}
+	return false; // step 3
+};