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/refs/json-schema-2019-09/meta/content.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "https://json-schema.org/draft/2019-09/schema",
+  "$id": "https://json-schema.org/draft/2019-09/meta/content",
+  "$vocabulary": {
+    "https://json-schema.org/draft/2019-09/vocab/content": true
+  },
+  "$recursiveAnchor": true,
+
+  "title": "Content vocabulary meta-schema",
+
+  "type": ["object", "boolean"],
+  "properties": {
+    "contentMediaType": {"type": "string"},
+    "contentEncoding": {"type": "string"},
+    "contentSchema": {"$recursiveRef": "#"}
+  }
+}