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/ToInt16.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var modulo = require('./modulo');
+var ToNumber = require('./ToNumber');
+var truncate = require('./truncate');
+
+var isFinite = require('math-intrinsics/isFinite');
+
+// https://262.ecma-international.org/14.0/#sec-toint16
+
+var two16 = 0x10000; // Math.pow(2, 16);
+
+module.exports = function ToInt16(argument) {
+	var number = ToNumber(argument);
+	if (!isFinite(number) || number === 0) {
+		return 0;
+	}
+	var int = truncate(number);
+	var int16bit = modulo(int, two16);
+	return int16bit >= 0x8000 ? int16bit - two16 : int16bit;
+};