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/ajv/lib/runtime/ucs2length.ts

@@ -0,0 +1,20 @@
+// https://mathiasbynens.be/notes/javascript-encoding
+// https://github.com/bestiejs/punycode.js - punycode.ucs2.decode
+export default function ucs2length(str: string): number {
+  const len = str.length
+  let length = 0
+  let pos = 0
+  let value: number
+  while (pos < len) {
+    length++
+    value = str.charCodeAt(pos++)
+    if (value >= 0xd800 && value <= 0xdbff && pos < len) {
+      // high surrogate, and there is a next character
+      value = str.charCodeAt(pos)
+      if ((value & 0xfc00) === 0xdc00) pos++ // low surrogate
+    }
+  }
+  return length
+}
+
+ucs2length.code = 'require("ajv/dist/runtime/ucs2length").default'