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/resolve/test/resolver/multirepo/package.json

@@ -0,0 +1,20 @@
+{
+  "name": "ljharb-monorepo-symlink-test",
+  "private": true,
+  "version": "0.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "postinstall": "lerna bootstrap",
+    "test": "node packages/package-a"
+  },
+  "author": "",
+  "license": "MIT",
+  "dependencies": {
+    "jquery": "^3.3.1",
+    "resolve": "../../../"
+  },
+  "devDependencies": {
+    "lerna": "^3.4.3"
+  }
+}