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/helpers/defaultEndianness.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $Uint8Array = GetIntrinsic('%Uint8Array%', true);
+var $Uint32Array = GetIntrinsic('%Uint32Array%', true);
+
+var typedArrayBuffer = require('typed-array-buffer');
+
+var uInt32 = $Uint32Array && new $Uint32Array([0x12345678]);
+var uInt8 = uInt32 && new $Uint8Array(typedArrayBuffer(uInt32));
+
+module.exports = uInt8
+	? uInt8[0] === 0x78
+		? 'little'
+		: uInt8[0] === 0x12
+			? 'big'
+			: uInt8[0] === 0x34
+				? 'mixed' // https://developer.mozilla.org/en-US/docs/Glossary/Endianness
+				: 'unknown' // ???
+	: 'indeterminate'; // no way to know