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/@esbuild/linux-x64/package.json

@@ -0,0 +1,20 @@
+{
+  "name": "@esbuild/linux-x64",
+  "version": "0.25.12",
+  "description": "The Linux 64-bit binary for esbuild, a JavaScript bundler.",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/evanw/esbuild.git"
+  },
+  "license": "MIT",
+  "preferUnplugged": true,
+  "engines": {
+    "node": ">=18"
+  },
+  "os": [
+    "linux"
+  ],
+  "cpu": [
+    "x64"
+  ]
+}