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/vite-plugin-pwa/client.d.ts

@@ -0,0 +1,6 @@
+import './vue.d.ts'
+import './preact.d.ts'
+import './react.d.ts'
+import './svelte.d.ts'
+import './solid.d.ts'
+import './vanillajs.d.ts'