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/2020/Number/signedRightShift.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var ToInt32 = require('../ToInt32');
+var ToUint32 = require('../ToUint32');
+
+// https://262.ecma-international.org/11.0/#sec-numeric-types-number-signedRightShift
+
+module.exports = function NumberSignedRightShift(x, y) {
+	if (typeof x !== 'number' || typeof y !== 'number') {
+		throw new $TypeError('Assertion failed: `x` and `y` arguments must be Numbers');
+	}
+
+	var lnum = ToInt32(x);
+	var rnum = ToUint32(y);
+
+	var shiftCount = rnum & 0x1F;
+
+	return lnum >> shiftCount;
+};