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/2019/ToInteger.js

@@ -0,0 +1,12 @@
+'use strict';
+
+var ES5ToInteger = require('../5/ToInteger');
+
+var ToNumber = require('./ToNumber');
+
+// https://262.ecma-international.org/6.0/#sec-tointeger
+
+module.exports = function ToInteger(value) {
+	var number = ToNumber(value);
+	return ES5ToInteger(number);
+};