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/2022/ToInt8.js

@@ -0,0 +1,10 @@
+'use strict';
+
+var ToUint8 = require('./ToUint8');
+
+// https://262.ecma-international.org/6.0/#sec-toint8
+
+module.exports = function ToInt8(argument) {
+	var int8bit = ToUint8(argument);
+	return int8bit >= 0x80 ? int8bit - 0x100 : int8bit;
+};