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/@rollup/rollup-linux-x64-gnu/README.md

@@ -0,0 +1,3 @@
+# `@rollup/rollup-linux-x64-gnu`
+
+This is the **x86_64-unknown-linux-gnu** binary for `rollup`

frontend/node_modules/@rollup/rollup-linux-x64-gnu/package.json

@@ -0,0 +1,25 @@
+{
+  "name": "@rollup/rollup-linux-x64-gnu",
+  "version": "4.62.2",
+  "os": [
+    "linux"
+  ],
+  "cpu": [
+    "x64"
+  ],
+  "files": [
+    "rollup.linux-x64-gnu.node"
+  ],
+  "description": "Native bindings for Rollup",
+  "author": "Lukas Taegert-Atkinson",
+  "homepage": "https://rollupjs.org/",
+  "license": "MIT",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/rollup/rollup.git"
+  },
+  "libc": [
+    "glibc"
+  ],
+  "main": "./rollup.linux-x64-gnu.node"
+}