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/terser/bin/uglifyjs

@@ -0,0 +1,10 @@
+#!/usr/bin/env node
+// -*- js -*-
+/* eslint-env node */
+
+"use strict";
+
+process.stderr.write( "DEPRECATION WARNING: uglifyjs binary will soon be discontinued!\n");
+process.stderr.write("Please use \"terser\" instead.\n\n");
+
+require("./terser");