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-weakset/index.js

@@ -0,0 +1,43 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+var callBound = require('call-bound');
+
+var $WeakSet = GetIntrinsic('%WeakSet%', true);
+
+/** @type {undefined | (<V>(thisArg: Set<V>, value: V) => boolean)} */
+var $setHas = callBound('WeakSet.prototype.has', true);
+
+if ($setHas) {
+	/** @type {undefined | (<K extends object, V>(thisArg: WeakMap<K, V>, key: K) => boolean)} */
+	var $mapHas = callBound('WeakMap.prototype.has', true);
+
+	/** @type {import('.')} */
+	module.exports = function isWeakSet(x) {
+		if (!x || typeof x !== 'object') {
+			return false;
+		}
+		try {
+			// @ts-expect-error TS can't figure out that $setHas is always truthy here
+			$setHas(x, $setHas);
+			if ($mapHas) {
+				try {
+					// @ts-expect-error this indeed might not be a weak collection
+					$mapHas(x, $mapHas);
+				} catch (e) {
+					return true;
+				}
+			}
+			// @ts-expect-error TS can't figure out that $WeakSet is always truthy here
+			return x instanceof $WeakSet; // core-js workaround, pre-v3
+		} catch (e) {}
+		return false;
+	};
+} else {
+	/** @type {import('.')} */
+	// @ts-expect-error
+	module.exports = function isWeakSet(x) { // eslint-disable-line no-unused-vars
+		// `WeakSet` does not exist, or does not have a `has` method
+		return false;
+	};
+}