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/crypto-random-string/index.d.ts

@@ -0,0 +1,17 @@
+/**
+Generate a [cryptographically strong](https://en.m.wikipedia.org/wiki/Strong_cryptography) random string.
+
+@param length - Length of the returned string.
+@returns A [`hex`](https://en.wikipedia.org/wiki/Hexadecimal) string.
+
+@example
+```
+import cryptoRandomString = require('crypto-random-string');
+
+cryptoRandomString(10);
+//=> '2cf05d94db'
+```
+*/
+declare function cryptoRandomString(length: number): string;
+
+export = cryptoRandomString;