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/dist/package.json

@@ -0,0 +1,10 @@
+{
+  "name": "dist",
+  "private": true,
+  "version": "1.0.0",
+  "main": "bundle.min.js",
+  "type": "commonjs",
+  "author": "",
+  "license": "BSD-2-Clause",
+  "description": "A package to hold the Terser dist bundle as commonjs while keeping the rest of it ESM. Nothing to see here."
+}