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/postcss/lib/comment.js

@@ -0,0 +1,13 @@
+'use strict'
+
+let Node = require('./node')
+
+class Comment extends Node {
+  constructor(defaults) {
+    super(defaults)
+    this.type = 'comment'
+  }
+}
+
+module.exports = Comment
+Comment.default = Comment