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/update-browserslist-db/README.md

@@ -0,0 +1,30 @@
+# Update Browserslist DB
+
+<img width="120" height="120" alt="Browserslist logo by Anton Popov"
+     src="https://browsersl.ist/logo.svg" align="right">
+
+CLI tool to update `caniuse-lite` with browsers DB
+from [Browserslist](https://github.com/browserslist/browserslist/) config.
+
+Some queries like `last 2 versions` or `>1%` depend on actual data
+from `caniuse-lite`.
+
+```sh
+npx update-browserslist-db@latest
+```
+Or if using `pnpm`:
+```sh
+pnpm exec update-browserslist-db latest
+```
+Or if using `yarn`:
+```sh
+yarn dlx update-browserslist-db@latest
+```
+
+<a href="https://evilmartians.com/?utm_source=update-browserslist-db">
+  <img src="https://evilmartians.com/badges/sponsored-by-evil-martians.svg"
+       alt="Sponsored by Evil Martians" width="236" height="54">
+</a>
+
+## Docs
+Read full docs **[here](https://github.com/browserslist/update-db#readme)**.