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/scheduler/package.json

@@ -0,0 +1,36 @@
+{
+  "name": "scheduler",
+  "version": "0.23.2",
+  "description": "Cooperative scheduler for the browser environment.",
+  "main": "index.js",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/facebook/react.git",
+    "directory": "packages/scheduler"
+  },
+  "license": "MIT",
+  "keywords": [
+    "react"
+  ],
+  "bugs": {
+    "url": "https://github.com/facebook/react/issues"
+  },
+  "homepage": "https://reactjs.org/",
+  "dependencies": {
+    "loose-envify": "^1.1.0"
+  },
+  "files": [
+    "LICENSE",
+    "README.md",
+    "index.js",
+    "unstable_mock.js",
+    "unstable_post_task.js",
+    "cjs/",
+    "umd/"
+  ],
+  "browserify": {
+    "transform": [
+      "loose-envify"
+    ]
+  }
+}