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/array-buffer-byte-length/index.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var callBound = require('call-bound');
+var $byteLength = callBound('ArrayBuffer.prototype.byteLength', true);
+
+var isArrayBuffer = require('is-array-buffer');
+
+/** @type {import('.')} */
+module.exports = function byteLength(ab) {
+	if (!isArrayBuffer(ab)) {
+		return NaN;
+	}
+	return $byteLength ? $byteLength(ab) : ab.byteLength;
+}; // in node < 0.11, byteLength is an own nonconfigurable property