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/index.d.ts

@@ -0,0 +1,26 @@
+import type { primitiveES6 } from './es2015';
+import es5 from './es5';
+import es6 from './es6';
+import es2015 from './es2015';
+
+declare function ToPrimitive(
+    input: unknown,
+    hint?: StringConstructor | NumberConstructor,
+): ToPrimitive.primitive;
+
+declare namespace ToPrimitive {
+    export type ES5 = typeof es5;
+    export const ES5: typeof es5;
+
+    /** @deprecated */
+    export type ES6 = typeof es6;
+    /** @deprecated */
+    export const ES6: typeof es6;
+
+    export type ES2015 = typeof es2015;
+    export const ES2015: typeof es2015;
+
+    export type primitive = primitiveES6 | bigint;
+}
+
+export = ToPrimitive;