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/dist/client/dev/svelte.js

@@ -0,0 +1,16 @@
+// src/client/dev/svelte.ts
+import { writable } from "svelte/store";
+function useRegisterSW(_options = {}) {
+  const needRefresh = writable(false);
+  const offlineReady = writable(false);
+  const updateServiceWorker = (_reloadPage) => {
+  };
+  return {
+    needRefresh,
+    offlineReady,
+    updateServiceWorker
+  };
+}
+export {
+  useRegisterSW
+};