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/2020/BigInt/divide.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $BigInt = GetIntrinsic('%BigInt%', true);
+var $RangeError = require('es-errors/range');
+var $TypeError = require('es-errors/type');
+
+// https://262.ecma-international.org/11.0/#sec-numeric-types-bigint-divide
+
+module.exports = function BigIntDivide(x, y) {
+	if (typeof x !== 'bigint' || typeof y !== 'bigint') {
+		throw new $TypeError('Assertion failed: `x` and `y` arguments must be BigInts');
+	}
+	if (y === $BigInt(0)) {
+		throw new $RangeError('Division by zero');
+	}
+	// shortcut for the actual spec mechanics
+	return x / y;
+};