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/babel-plugin-polyfill-corejs3/core-js-compat/README.md

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+`core-js-compat` exposes some files as JSON, and they cannot be
+imported by Node.js ESM files.
+This folder proxies `core-js-compat` to ensure that every entry
+is CJS and can be safely imported.

frontend/node_modules/babel-plugin-polyfill-corejs3/core-js-compat/data.js

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+module.exports = require("core-js-compat/data");

frontend/node_modules/babel-plugin-polyfill-corejs3/core-js-compat/entries.js

@@ -0,0 +1 @@
+module.exports = require("core-js-compat/entries");

frontend/node_modules/babel-plugin-polyfill-corejs3/core-js-compat/get-modules-list-for-target-version.js

@@ -0,0 +1 @@
+module.exports = require("core-js-compat/get-modules-list-for-target-version");