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/2020/thisNumberValue.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+var $NumberValueOf = callBound('Number.prototype.valueOf');
+
+// https://262.ecma-international.org/6.0/#sec-properties-of-the-number-prototype-object
+
+module.exports = function thisNumberValue(value) {
+	if (typeof value === 'number') {
+		return value;
+	}
+
+	return $NumberValueOf(value);
+};
+