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/which-boxed-primitive/index.js

@@ -0,0 +1,31 @@
+'use strict';
+
+var isString = require('is-string');
+var isNumber = require('is-number-object');
+var isBoolean = require('is-boolean-object');
+var isSymbol = require('is-symbol');
+var isBigInt = require('is-bigint');
+
+/** @type {import('.')} */
+// eslint-disable-next-line consistent-return
+module.exports = function whichBoxedPrimitive(value) {
+	// eslint-disable-next-line eqeqeq
+	if (value == null || (typeof value !== 'object' && typeof value !== 'function')) {
+		return null;
+	}
+	if (isString(value)) {
+		return 'String';
+	}
+	if (isNumber(value)) {
+		return 'Number';
+	}
+	if (isBoolean(value)) {
+		return 'Boolean';
+	}
+	if (isSymbol(value)) {
+		return 'Symbol';
+	}
+	if (isBigInt(value)) {
+		return 'BigInt';
+	}
+};