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/math-intrinsics/isFinite.js

@@ -0,0 +1,12 @@
+'use strict';
+
+var $isNaN = require('./isNaN');
+
+/** @type {import('./isFinite')} */
+module.exports = function isFinite(x) {
+	return (typeof x === 'number' || typeof x === 'bigint')
+        && !$isNaN(x)
+        && x !== Infinity
+        && x !== -Infinity;
+};
+