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/has-proto/accessor.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var result = require('./')();
+
+var test = {
+	__proto__: null,
+	foo: {}
+};
+
+/** @type {import('./accessor')} */
+module.exports = function hasAccessor() {
+	/* eslint no-proto: 0 */
+	return result
+		&& !('toString' in test)
+		// eslint-disable-next-line no-extra-parens
+		&& /** @type {{ __proto__?: typeof Object.prototype }} */ ({}).__proto__ === Object.prototype
+		// eslint-disable-next-line no-extra-parens
+		&& /** @type {ReadonlyArray<never> & { __proto__?: typeof Array.prototype }} */ (
+			[]).__proto__ === Array.prototype;
+};