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/clamp.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var max = require('math-intrinsics/max');
+var min = require('math-intrinsics/min');
+
+// https://262.ecma-international.org/12.0/#clamping
+
+module.exports = function clamp(x, lower, upper) {
+	if (typeof x !== 'number' || typeof lower !== 'number' || typeof upper !== 'number' || !(lower <= upper)) {
+		throw new $TypeError('Assertion failed: all three arguments must be MVs, and `lower` must be `<= upper`');
+	}
+	return min(max(lower, x), upper);
+};