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/filelist/package.json

@@ -0,0 +1,28 @@
+{
+  "name": "filelist",
+  "version": "1.0.6",
+  "description": "Lazy-evaluating list of files, based on globs or regex patterns",
+  "main": "index.js",
+  "types": "index.d.ts",
+  "scripts": {
+    "test": "jake test"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git://github.com/mde/filelist.git"
+  },
+  "keywords": [
+    "file",
+    "utility",
+    "glob"
+  ],
+  "author": "Matthew Eernisse <mde@fleegix.org> (http://fleegix.org)",
+  "license": "Apache-2.0",
+  "bugs": {
+    "url": "https://github.com/mde/filelist/issues"
+  },
+  "homepage": "https://github.com/mde/filelist",
+  "dependencies": {
+    "minimatch": "^5.0.1"
+  }
+}