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/which-collection/index.d.ts

@@ -0,0 +1,13 @@
+/**
+ * Determines the type of the given collection, or returns false.
+ *
+ * @param {unknown} value The potential collection
+ * @returns {'Map' | 'Set' | 'WeakMap' | 'WeakSet' | false} 'Map' | 'Set' | 'WeakMap' | 'WeakSet' | false
+ */
+declare function whichCollection<K, V>(value: Map<K, V>): 'Map';
+declare function whichCollection<T>(value: Set<T>): 'Set';
+declare function whichCollection<K extends WeakKey, V>(value: WeakMap<K, V>): 'WeakMap';
+declare function whichCollection<T extends WeakKey>(value: WeakSet<T>): 'WeakSet';
+declare function whichCollection(value: null | undefined | boolean | number | bigint | symbol | unknown): false;
+
+export = whichCollection;