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/2022/floor.js

@@ -0,0 +1,14 @@
+'use strict';
+
+// var modulo = require('./modulo');
+var $floor = require('math-intrinsics/floor');
+
+// http://262.ecma-international.org/11.0/#eqn-floor
+
+module.exports = function floor(x) {
+	// return x - modulo(x, 1);
+	if (typeof x === 'bigint') {
+		return x;
+	}
+	return $floor(x);
+};