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/2023/ParseHexOctet.js

@@ -0,0 +1,40 @@
+'use strict';
+
+var $SyntaxError = require('es-errors/syntax');
+var $TypeError = require('es-errors/type');
+
+var substring = require('./substring');
+
+var isInteger = require('math-intrinsics/isInteger');
+var isNaN = require('math-intrinsics/isNaN');
+
+// https://262.ecma-international.org/14.0/#sec-parsehexoctet
+
+module.exports = function ParseHexOctet(string, position) {
+	if (typeof string !== 'string') {
+		throw new $TypeError('Assertion failed: `string` must be a String');
+	}
+	if (!isInteger(position) || position < 0) {
+		throw new $TypeError('Assertion failed: `position` must be a nonnegative integer');
+	}
+
+	var len = string.length; // step 1
+	if ((position + 2) > len) { // step 2
+		var error = new $SyntaxError('requested a position on a string that does not contain 2 characters at that position'); // step 2.a
+		return [error]; // step 2.b
+	}
+	var hexDigits = substring(string, position, position + 2); // step 3
+
+	var n = +('0x' + hexDigits);
+	if (isNaN(n)) {
+		return [new $SyntaxError('Invalid hexadecimal characters')];
+	}
+	return n;
+
+	/*
+	4. Let _parseResult_ be ParseText(StringToCodePoints(_hexDigits_), |HexDigits[~Sep]|).
+    5. If _parseResult_ is not a Parse Node, return _parseResult_.
+    6. Let _n_ be the unsigned 8-bit value corresponding with the MV of _parseResult_.
+    7. Return _n_.
+    */
+};