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/unbox-primitive/index.d.ts

@@ -0,0 +1,13 @@
+declare function unboxPrimitive<T extends unboxPrimitive.Boxed>(value: T): unboxPrimitive.Unbox<T>;
+
+declare namespace unboxPrimitive {
+    type Boxed = String | Number | Boolean | Symbol | BigInt;
+    type Unbox<T extends Boxed> = T extends String ? string
+        : T extends Number ? number
+        : T extends Boolean ? boolean
+        : T extends Symbol ? symbol
+        : T extends BigInt ? bigint
+        : never;
+}
+
+export = unboxPrimitive;