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/vocabularies/validation/limitContains.ts

@@ -0,0 +1,16 @@
+import type {CodeKeywordDefinition} from "../../types"
+import type {KeywordCxt} from "../../compile/validate"
+import {checkStrictMode} from "../../compile/util"
+
+const def: CodeKeywordDefinition = {
+  keyword: ["maxContains", "minContains"],
+  type: "array",
+  schemaType: "number",
+  code({keyword, parentSchema, it}: KeywordCxt) {
+    if (parentSchema.contains === undefined) {
+      checkStrictMode(it, `"${keyword}" without "contains" is ignored`)
+    }
+  },
+}
+
+export default def