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/is-symbol/index.js

@@ -0,0 +1,40 @@
+'use strict';
+
+var callBound = require('call-bound');
+var $toString = callBound('Object.prototype.toString');
+var hasSymbols = require('has-symbols')();
+var safeRegexTest = require('safe-regex-test');
+
+if (hasSymbols) {
+	var $symToStr = callBound('Symbol.prototype.toString');
+	var isSymString = safeRegexTest(/^Symbol\(.*\)$/);
+
+	/** @type {(value: object) => value is Symbol} */
+	var isSymbolObject = function isRealSymbolObject(value) {
+		if (typeof value.valueOf() !== 'symbol') {
+			return false;
+		}
+		return isSymString($symToStr(value));
+	};
+
+	/** @type {import('.')} */
+	module.exports = function isSymbol(value) {
+		if (typeof value === 'symbol') {
+			return true;
+		}
+		if (!value || typeof value !== 'object' || $toString(value) !== '[object Symbol]') {
+			return false;
+		}
+		try {
+			return isSymbolObject(value);
+		} catch (e) {
+			return false;
+		}
+	};
+} else {
+	/** @type {import('.')} */
+	module.exports = function isSymbol(value) {
+		// this environment does not support Symbols.
+		return false && value;
+	};
+}