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/index.js

@@ -0,0 +1,15 @@
+'use strict';
+
+var test = {
+	__proto__: null,
+	foo: {}
+};
+
+// @ts-expect-error: TS errors on an inherited property for some reason
+var result = { __proto__: test }.foo === test.foo
+	&& !(test instanceof Object);
+
+/** @type {import('.')} */
+module.exports = function hasProto() {
+	return result;
+};