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/ValidateAtomicAccess.js

@@ -0,0 +1,40 @@
+'use strict';
+
+var $RangeError = require('es-errors/range');
+var $TypeError = require('es-errors/type');
+
+var ToIndex = require('./ToIndex');
+var TypedArrayElementSize = require('./TypedArrayElementSize');
+
+var isTypedArray = require('is-typed-array');
+var typedArrayByteOffset = require('typed-array-byte-offset');
+var typedArrayLength = require('typed-array-length');
+
+// https://262.ecma-international.org/13.0/#sec-validateatomicaccess
+
+module.exports = function ValidateAtomicAccess(typedArray, requestIndex) {
+	if (!isTypedArray(typedArray)) {
+		throw new $TypeError('Assertion failed: `typedArray` must be a TypedArray');
+	}
+
+	var length = typedArrayLength(typedArray); // step 1
+
+	var accessIndex = ToIndex(requestIndex); // step 2
+
+	/*
+	// this assertion can never be reached
+	if (!(accessIndex >= 0)) {
+		throw new $TypeError('Assertion failed: accessIndex >= 0'); // step 4
+	}
+	*/
+
+	if (accessIndex >= length) {
+		throw new $RangeError('index out of range'); // step 4
+	}
+
+	var elementSize = TypedArrayElementSize(typedArray); // step 5
+
+	var offset = typedArrayByteOffset(typedArray); // step 6
+
+	return (accessIndex * elementSize) + offset; // step 7
+};