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/browserslist/error.js

@@ -0,0 +1,12 @@
+function BrowserslistError(message) {
+  this.name = 'BrowserslistError'
+  this.message = message
+  this.browserslist = true
+  if (Error.captureStackTrace) {
+    Error.captureStackTrace(this, BrowserslistError)
+  }
+}
+
+BrowserslistError.prototype = Error.prototype
+
+module.exports = BrowserslistError