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/2024/ToLength.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var MAX_SAFE_INTEGER = require('math-intrinsics/constants/maxSafeInteger');
+
+var ToIntegerOrInfinity = require('./ToIntegerOrInfinity');
+
+// https://262.ecma-international.org/12.0/#sec-tolength
+
+module.exports = function ToLength(argument) {
+	var len = ToIntegerOrInfinity(argument);
+	if (len <= 0) { return 0; } // includes converting -0 to +0
+	if (len > MAX_SAFE_INTEGER) { return MAX_SAFE_INTEGER; }
+	return len;
+};