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/ToUint8.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var ToNumber = require('./ToNumber');
+
+var $isNaN = require('math-intrinsics/isNaN');
+var $isFinite = require('math-intrinsics/isFinite');
+var $sign = require('math-intrinsics/sign');
+var abs = require('math-intrinsics/abs');
+var floor = require('math-intrinsics/floor');
+var modulo = require('math-intrinsics/mod');
+
+// https://262.ecma-international.org/6.0/#sec-touint8
+
+module.exports = function ToUint8(argument) {
+	var number = ToNumber(argument);
+	if ($isNaN(number) || number === 0 || !$isFinite(number)) { return 0; }
+	var posInt = $sign(number) * floor(abs(number));
+	return modulo(posInt, 0x100);
+};