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/object.assign/shim.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var define = require('define-properties');
+var getPolyfill = require('./polyfill');
+
+module.exports = function shimAssign() {
+	var polyfill = getPolyfill();
+	define(
+		Object,
+		{ assign: polyfill },
+		{ assign: function () { return Object.assign !== polyfill; } }
+	);
+	return polyfill;
+};