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/es-abstract/2024/IsStrictlyEqual.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var SameValueNonNumber = require('./SameValueNonNumber');
+var Type = require('./Type');
+var NumberEqual = require('./Number/equal');
+
+// https://262.ecma-international.org/14.0/#sec-isstrictlyequal
+
+module.exports = function IsStrictlyEqual(x, y) {
+	if (Type(x) !== Type(y)) {
+		return false;
+	}
+	return typeof x === 'number' ? NumberEqual(x, y) : SameValueNonNumber(x, y);
+};