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/test/index.js

@@ -0,0 +1,28 @@
+'use strict';
+
+var test = require('tape');
+var gPO = require('reflect.getprototypeof/polyfill')();
+var gOPD = require('gopd');
+
+var hasProto = require('../');
+
+var getter = require('dunder-proto/get');
+
+test('hasProto', function (t) {
+	var result = hasProto();
+	t.equal(typeof result, 'boolean', 'returns a boolean (' + result + ')');
+
+	var obj = { __proto__: null };
+	if (result) {
+		t.notOk('toString' in obj, 'null object lacks toString');
+		if (gOPD && getter) {
+			t.equal(getter(obj), null);
+		}
+	} else {
+		t.ok('toString' in obj, 'without proto, null object has toString');
+		t.equal(gPO(obj), Object.prototype);
+	}
+
+	t.end();
+});
+