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/json-schema-traverse/index.d.ts

@@ -0,0 +1,40 @@
+declare function traverse(
+  schema: traverse.SchemaObject,
+  opts: traverse.Options,
+  cb?: traverse.Callback
+): void;
+
+declare function traverse(
+  schema: traverse.SchemaObject,
+  cb: traverse.Callback
+): void;
+
+declare namespace traverse {
+  interface SchemaObject {
+    $id?: string;
+    $schema?: string;
+    [x: string]: any;
+  }
+
+  type Callback = (
+    schema: SchemaObject,
+    jsonPtr: string,
+    rootSchema: SchemaObject,
+    parentJsonPtr?: string,
+    parentKeyword?: string,
+    parentSchema?: SchemaObject,
+    keyIndex?: string | number
+  ) => void;
+
+  interface Options {
+    allKeys?: boolean;
+    cb?:
+      | Callback
+      | {
+          pre?: Callback;
+          post?: Callback;
+        };
+  }
+}
+
+export = traverse;