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/@babel/helper-globals/data/builtin-upper.json

@@ -0,0 +1,51 @@
+[
+  "AggregateError",
+  "Array",
+  "ArrayBuffer",
+  "Atomics",
+  "BigInt",
+  "BigInt64Array",
+  "BigUint64Array",
+  "Boolean",
+  "DataView",
+  "Date",
+  "Error",
+  "EvalError",
+  "FinalizationRegistry",
+  "Float16Array",
+  "Float32Array",
+  "Float64Array",
+  "Function",
+  "Infinity",
+  "Int16Array",
+  "Int32Array",
+  "Int8Array",
+  "Intl",
+  "Iterator",
+  "JSON",
+  "Map",
+  "Math",
+  "NaN",
+  "Number",
+  "Object",
+  "Promise",
+  "Proxy",
+  "RangeError",
+  "ReferenceError",
+  "Reflect",
+  "RegExp",
+  "Set",
+  "SharedArrayBuffer",
+  "String",
+  "Symbol",
+  "SyntaxError",
+  "TypeError",
+  "Uint16Array",
+  "Uint32Array",
+  "Uint8Array",
+  "Uint8ClampedArray",
+  "URIError",
+  "WeakMap",
+  "WeakRef",
+  "WeakSet"
+]