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/regexpu-core/rewrite-pattern.d.ts

@@ -0,0 +1,17 @@
+declare module "regexpu-core" {
+  export type RegexpuOptions = {
+    unicodeFlag?: "transform" | false;
+    unicodeSetsFlag?: "transform" | false;
+    dotAllFlag?: "transform" | false;
+    unicodePropertyEscapes?: "transform" | false;
+    namedGroups?: "transform" | false;
+    onNamedGroup?: (name: string, index: number) => void;
+    modifiers?: "transform" | false | "parse";
+    onNewFlags?: (flags: string) => void;
+  };
+  export default function rewritePattern(
+    pattern: string,
+    flags: string,
+    options: RegexpuOptions | undefined
+  ): string;
+}