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/intToBinaryString.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var $floor = require('math-intrinsics/floor');
+
+// https://runestone.academy/ns/books/published/pythonds/BasicDS/ConvertingDecimalNumberstoBinaryNumbers.html#:~:text=The%20Divide%20by%202%20algorithm,have%20a%20remainder%20of%200
+
+module.exports = function intToBinaryString(x) {
+	var str = '';
+	var y;
+
+	while (x > 0) {
+		y = x / 2;
+		x = $floor(y); // eslint-disable-line no-param-reassign
+		if (y === x) {
+			str = '0' + str;
+		} else {
+			str = '1' + str;
+		}
+	}
+	return str;
+};