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/object-keys/isArguments.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var toStr = Object.prototype.toString;
+
+module.exports = function isArguments(value) {
+	var str = toStr.call(value);
+	var isArgs = str === '[object Arguments]';
+	if (!isArgs) {
+		isArgs = str !== '[object Array]' &&
+			value !== null &&
+			typeof value === 'object' &&
+			typeof value.length === 'number' &&
+			value.length >= 0 &&
+			toStr.call(value.callee) === '[object Function]';
+	}
+	return isArgs;
+};