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/SetDataHas.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var CanonicalizeKeyedCollectionKey = require('./CanonicalizeKeyedCollectionKey');
+var IsArray = require('./IsArray');
+var SameValue = require('./SameValue');
+
+var some = require('../helpers/some');
+
+// https://262.ecma-international.org/16.0/#sec-setdatahas
+
+module.exports = function SetDataHas(setData, value) {
+	if (!IsArray(setData) && setData !== 'EMPTY') {
+		throw new $TypeError('Assertion failed: `setData` must be a List or ~EMPTY~');
+	}
+
+	// if (SetDataIndex(setData, value) === 'NOT-FOUND') { return false; } // step 1
+	// return true; // step 2
+
+	var canonValue = CanonicalizeKeyedCollectionKey(value);
+
+	return some(setData, function (e) {
+		return SameValue(e, canonValue);
+	});
+};