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/fast-uri/benchmark/string-array-to-hex-stripped.mjs

@@ -0,0 +1,24 @@
+import { Bench } from 'tinybench'
+import { stringArrayToHexStripped } from '../lib/utils.js'
+
+const benchStringArrayToHexStripped = new Bench({ name: 'stringArrayToHexStripped' })
+
+const case1 = ['0', '0', '0', '0']
+const case2 = ['0', '0', '0', '1']
+const case3 = ['0', '0', '1', '0']
+const case4 = ['0', '1', '0', '0']
+const case5 = ['1', '0', '0', '0']
+const case6 = ['1', '0', '0', '1']
+
+benchStringArrayToHexStripped.add('stringArrayToHexStripped', function () {
+  stringArrayToHexStripped(case1)
+  stringArrayToHexStripped(case2)
+  stringArrayToHexStripped(case3)
+  stringArrayToHexStripped(case4)
+  stringArrayToHexStripped(case5)
+  stringArrayToHexStripped(case6)
+})
+
+await benchStringArrayToHexStripped.run()
+console.log(benchStringArrayToHexStripped.name)
+console.table(benchStringArrayToHexStripped.table())