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/resolve/lib/core.js

@@ -0,0 +1,12 @@
+'use strict';
+
+var isCoreModule = require('is-core-module');
+var data = require('./core.json');
+
+var core = {};
+for (var mod in data) { // eslint-disable-line no-restricted-syntax
+    if (Object.prototype.hasOwnProperty.call(data, mod)) {
+        core[mod] = isCoreModule(mod);
+    }
+}
+module.exports = core;