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/which-collection/index.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var isMap = require('is-map');
+var isSet = require('is-set');
+var isWeakMap = require('is-weakmap');
+var isWeakSet = require('is-weakset');
+
+/** @type {import('.')} */
+module.exports = function whichCollection(/** @type {unknown} */ value) {
+	if (value && typeof value === 'object') {
+		if (isMap(value)) {
+			return 'Map';
+		}
+		if (isSet(value)) {
+			return 'Set';
+		}
+		if (isWeakMap(value)) {
+			return 'WeakMap';
+		}
+		if (isWeakSet(value)) {
+			return 'WeakSet';
+		}
+	}
+	return false;
+};