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/tools/exit.cjs

@@ -0,0 +1,7 @@
+// workaround for tty output truncation upon process.exit()
+// https://github.com/nodejs/node/issues/6456
+
+[process.stdout, process.stderr].forEach((s) => {
+  s && s.isTTY && s._handle && s._handle.setBlocking &&
+    s._handle.setBlocking(true)
+});