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/json5/lib/parse.d.ts

@@ -0,0 +1,15 @@
+/**
+ * Parses a JSON5 string, constructing the JavaScript value or object described
+ * by the string.
+ * @template T The type of the return value.
+ * @param text The string to parse as JSON5.
+ * @param reviver A function that prescribes how the value originally produced
+ * by parsing is transformed before being returned.
+ * @returns The JavaScript value converted from the JSON5 string.
+ */
+declare function parse<T = any>(
+    text: string,
+    reviver?: ((this: any, key: string, value: any) => any) | null,
+): T
+
+export = parse