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/2025/GeneratorValidate.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var SLOT = require('internal-slot');
+
+// https://262.ecma-international.org/16.0/#sec-generatorvalidate
+
+module.exports = function GeneratorValidate(generator, generatorBrand) {
+	SLOT.assert(generator, '[[GeneratorState]]'); // step 1
+	SLOT.assert(generator, '[[GeneratorBrand]]'); // step 2
+
+	var brand = SLOT.get(generator, '[[GeneratorBrand]]');
+	if (brand !== generatorBrand) {
+		throw new $TypeError('Assertion failed: generator brand is unexpected: ' + brand);
+	}
+	SLOT.assert(generator, '[[GeneratorContext]]'); // step 4
+	var state = SLOT.get(generator, '[[GeneratorState]]'); // step 5
+	if (state === 'EXECUTING') {
+		throw new $TypeError('generator is executing');
+	}
+
+	return state; // step 7
+};