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/is-shared-array-buffer/index.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+/** @type {undefined | ((thisArg: SharedArrayBuffer) => number)} */
+var $byteLength = callBound('SharedArrayBuffer.prototype.byteLength', true);
+
+/** @type {import('.')} */
+module.exports = $byteLength
+	? function isSharedArrayBuffer(obj) {
+		if (!obj || typeof obj !== 'object') {
+			return false;
+		}
+		try {
+			// @ts-expect-error TS can't figure out this closed-over variable is non-nullable, and it's fine that `obj` might not be a SAB
+			$byteLength(obj);
+			return true;
+		} catch (e) {
+			return false;
+		}
+	}
+	: function isSharedArrayBuffer(_obj) { // eslint-disable-line no-unused-vars
+		return false;
+	};