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/es5.d.ts

@@ -0,0 +1,17 @@
+declare function ToPrimitive(
+    input: ToPrimitive.unknownES5,
+    hint?: StringConstructor | NumberConstructor,
+): ToPrimitive.primitiveES5;
+
+declare namespace ToPrimitive {
+    export type primitiveES5 =
+        | null
+        | undefined
+        | string
+        | number
+        | boolean
+    ;
+    export type unknownES5 = primitiveES5 | object;
+}
+
+export = ToPrimitive;