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-finalizationregistry/index.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+/** @type {undefined | ((value: ThisParameterType<typeof FinalizationRegistry.prototype.register>, ...args: Parameters<typeof FinalizationRegistry.prototype.register>) => ReturnType<typeof FinalizationRegistry.prototype.register>)} */
+var $register = callBound('FinalizationRegistry.prototype.register', true);
+
+/** @type {import('.')} */
+module.exports = $register
+	? function isFinalizationRegistry(value) {
+		if (!value || typeof value !== 'object') {
+			return false;
+		}
+		try {
+			// @ts-expect-error TS can't figure out that it's always truthy here
+			$register(value, {}, null);
+			return true;
+		} catch (e) {
+			return false;
+		}
+	}
+	// @ts-ignore unused var
+	: function isFinalizationRegistry(value) { // eslint-disable-line no-unused-vars
+		return false;
+	};