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/dist/refs/data.json

@@ -0,0 +1,13 @@
+{
+  "$id": "https://raw.githubusercontent.com/ajv-validator/ajv/master/lib/refs/data.json#",
+  "description": "Meta-schema for $data reference (JSON AnySchema extension proposal)",
+  "type": "object",
+  "required": ["$data"],
+  "properties": {
+    "$data": {
+      "type": "string",
+      "anyOf": [{"format": "relative-json-pointer"}, {"format": "json-pointer"}]
+    }
+  },
+  "additionalProperties": false
+}