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-to-primitive/index.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var defineProperty = require('es-define-property');
+
+var ES5 = require('./es5');
+var ES6 = require('./es6');
+var ES2015 = require('./es2015');
+
+module.exports = ES2015;
+
+// `module.exports.x =` form lets `cjs-module-lexer` statically detect these as named exports (ESM interop / attw)
+module.exports.ES5 = ES5;
+module.exports.ES6 = ES6;
+module.exports.ES2015 = ES2015;
+
+if (defineProperty) {
+	// `ES2015` is `module.exports`; defining here rather than on `module.exports` keeps the lexer from de-listing the above
+	defineProperty(ES2015, 'ES5', { enumerable: false });
+	defineProperty(ES2015, 'ES6', { enumerable: false });
+	defineProperty(ES2015, 'ES2015', { enumerable: false });
+}