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/es-to-primitive/es2015.d.ts

@@ -0,0 +1,14 @@
+import type { primitiveES5 } from './es5';
+
+declare function ToPrimitive(
+    input: unknown,
+    hint?: StringConstructor | NumberConstructor,
+): ToPrimitive.primitiveES6;
+
+
+declare namespace ToPrimitive {
+    export type primitiveES6 = primitiveES5 | symbol;
+    export type unknownES6 = primitiveES6 | object;
+}
+
+export = ToPrimitive;