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/mutator.js

@@ -0,0 +1,33 @@
+'use strict';
+
+var result = require('./')();
+
+var test = {
+	__proto__: null,
+	foo: {}
+};
+
+var setter = require('dunder-proto/set');
+
+/** @type {import('./mutator')} */
+module.exports = function hasMutator() {
+	if (!result) {
+		return false;
+	}
+
+	var obj = { __proto__: test };
+	// @ts-expect-error: TS errors on an inherited property for some reason
+	if (obj.foo !== test.foo) {
+		return false;
+	}
+
+	if (!setter) {
+		return false;
+	}
+
+	setter(obj, null);
+	if ('foo' in obj || 'toString' in obj) {
+		return false;
+	}
+	return true;
+};