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/MakeFullYear.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var ToIntegerOrInfinity = require('./ToIntegerOrInfinity');
+
+var isNaN = require('math-intrinsics/isNaN');
+
+// https://262.ecma-international.org/15.0/#sec-makefullyear
+
+module.exports = function MakeFullYear(year) {
+	if (typeof year !== 'number') {
+		throw new $TypeError('Assertion failed: `year` must be a Number');
+	}
+
+	if (isNaN(year)) {
+		return NaN; // step 1
+	}
+
+	var truncated = ToIntegerOrInfinity(year); // step 2
+	if (0 <= truncated && truncated <= 99) {
+		return 1900 + truncated; // step 3
+	}
+
+	return truncated; // step 4
+};