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/core-js-compat/shared.d.ts

@@ -0,0 +1,34 @@
+export type ModuleName = string;
+
+export type Target =
+  | 'android'
+  | 'bun'
+  | 'chrome'
+  | 'chrome-android'
+  | 'deno'
+  | 'edge'
+  | 'electron'
+  | 'firefox'
+  | 'firefox-android'
+  | 'hermes'
+  | 'ie'
+  | 'ios'
+  | 'node'
+  | 'opera'
+  | 'opera-android'
+  | 'phantom'
+  | 'quest'
+  | 'react-native'
+  | 'rhino'
+  | 'safari'
+  | 'samsung'
+  /** `quest` alias */
+  | 'oculus'
+  /** `react-native` alias */
+  | 'react'
+  /** `react-native` alias */
+  | 'reactnative'
+  /** @deprecated use `opera-android` instead */
+  | 'opera_mobile';
+
+export type TargetVersion = string;