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/unicode-match-property-ecmascript/index.js

@@ -0,0 +1,16 @@
+'use strict';
+
+const canonicalProperties = require('unicode-canonical-property-names-ecmascript');
+const propertyAliases = require('unicode-property-aliases-ecmascript');
+
+const matchProperty = function(property) {
+	if (canonicalProperties.has(property)) {
+		return property;
+	}
+	if (propertyAliases.has(property)) {
+		return propertyAliases.get(property);
+	}
+	throw new Error(`Unknown property: ${ property }`);
+};
+
+module.exports = matchProperty;