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/IsFixedLengthArrayBuffer.js

@@ -0,0 +1,29 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var callBound = require('call-bound');
+
+var $arrayBufferResizable = callBound('%ArrayBuffer.prototype.resizable%', true);
+var $sharedArrayGrowable = callBound('%SharedArrayBuffer.prototype.growable%', true);
+
+var isArrayBuffer = require('is-array-buffer');
+var isSharedArrayBuffer = require('is-shared-array-buffer');
+
+// https://262.ecma-international.org/15.0/#sec-isfixedlengtharraybuffer
+
+module.exports = function IsFixedLengthArrayBuffer(arrayBuffer) {
+	var isAB = isArrayBuffer(arrayBuffer);
+	var isSAB = isSharedArrayBuffer(arrayBuffer);
+	if (!isAB && !isSAB) {
+		throw new $TypeError('Assertion failed: `arrayBuffer` must be an ArrayBuffer or SharedArrayBuffer');
+	}
+
+	if (isAB && $arrayBufferResizable) {
+		return !$arrayBufferResizable(arrayBuffer); // step 1
+	}
+	if (isSAB && $sharedArrayGrowable) {
+		return !$sharedArrayGrowable(arrayBuffer); // step 1
+	}
+	return true; // step 2
+};