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/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "benchmark",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "private": true,
+  "scripts": {
+    "bench": "node benchmark.mjs"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "tinybench": "^5.0.0",
+    "uri-js": "^4.4.1"
+  }
+}