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/Number/multiply.js

@@ -0,0 +1,30 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isFinite = require('math-intrinsics/isFinite');
+
+var isNaN = require('../../helpers/isNaN');
+
+// https://262.ecma-international.org/11.0/#sec-numeric-types-number-multiply
+
+module.exports = function NumberMultiply(x, y) {
+	if (typeof x !== 'number' || typeof y !== 'number') {
+		throw new $TypeError('Assertion failed: `x` and `y` arguments must be Numbers');
+	}
+
+	if (isNaN(x) || isNaN(y) || (x === 0 && !isFinite(y)) || (!isFinite(x) && y === 0)) {
+		return NaN;
+	}
+	if (!isFinite(x) && !isFinite(y)) {
+		return x === y ? Infinity : -Infinity;
+	}
+	if (!isFinite(x) && y !== 0) {
+		return x > 0 ? Infinity : -Infinity;
+	}
+	if (!isFinite(y) && x !== 0) {
+		return y > 0 ? Infinity : -Infinity;
+	}
+
+	// shortcut for the actual spec mechanics
+	return x * y;
+};