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/async-function/legacy.js

@@ -0,0 +1,18 @@
+'use strict';
+
+/** @type {import('.').AsyncFunctionConstructor | false} */
+var cached;
+
+/** @type {import('.')} */
+module.exports = function getAsyncFunction() {
+	if (typeof cached === 'undefined') {
+		try {
+			// eslint-disable-next-line no-new-func
+			cached = Function('return async function () {}')().constructor;
+		} catch (e) {
+			cached = false;
+		}
+	}
+	return cached;
+};
+