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/json-schema-2019-09/meta/meta-data.json

@@ -0,0 +1,37 @@
+{
+  "$schema": "https://json-schema.org/draft/2019-09/schema",
+  "$id": "https://json-schema.org/draft/2019-09/meta/meta-data",
+  "$vocabulary": {
+    "https://json-schema.org/draft/2019-09/vocab/meta-data": true
+  },
+  "$recursiveAnchor": true,
+
+  "title": "Meta-data vocabulary meta-schema",
+
+  "type": ["object", "boolean"],
+  "properties": {
+    "title": {
+      "type": "string"
+    },
+    "description": {
+      "type": "string"
+    },
+    "default": true,
+    "deprecated": {
+      "type": "boolean",
+      "default": false
+    },
+    "readOnly": {
+      "type": "boolean",
+      "default": false
+    },
+    "writeOnly": {
+      "type": "boolean",
+      "default": false
+    },
+    "examples": {
+      "type": "array",
+      "items": true
+    }
+  }
+}