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/2025/SetDataSize.js

@@ -0,0 +1,30 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var forEach = require('../helpers/forEach');
+var isArray = require('../helpers/IsArray');
+
+// https://262.ecma-international.org/16.0/#sec-setdatasize
+
+// TODO: when spec enums are unforgeable, uncomment ~EMPTY~ check
+
+module.exports = function SetDataSize(setData) {
+	if (!isArray(setData) && setData !== 'EMPTY') {
+		throw new $TypeError('Assertion failed: `setData` must be a List or ~EMPTY~');
+	}
+
+	if (setData === 'EMPTY') {
+		return 0;
+	}
+
+	var count = 0; // step 1
+
+	forEach(setData, function (e, i) { // step 2
+		if (i in setData /* && e !== ~EMPTY~ */) {
+			count += 1; // step 2.a
+		}
+	});
+
+	return count; // step 3
+};