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/2016/IsPromise.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+var $PromiseThen = callBound('Promise.prototype.then', true);
+
+var isObject = require('es-object-atoms/isObject');
+
+// https://262.ecma-international.org/6.0/#sec-ispromise
+
+module.exports = function IsPromise(x) {
+	if (!isObject(x)) {
+		return false;
+	}
+	if (!$PromiseThen) { // Promises are not supported
+		return false;
+	}
+	try {
+		$PromiseThen(x); // throws if not a promise
+	} catch (e) {
+		return false;
+	}
+	return true;
+};