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/runtime/regenerator/index.js

@@ -0,0 +1,15 @@
+// TODO(Babel 8): Remove this file.
+
+var runtime = require("../helpers/regeneratorRuntime")();
+module.exports = runtime;
+
+// Copied from https://github.com/facebook/regenerator/blob/main/packages/runtime/runtime.js#L736=
+try {
+  regeneratorRuntime = runtime;
+} catch (accidentalStrictMode) {
+  if (typeof globalThis === "object") {
+    globalThis.regeneratorRuntime = runtime;
+  } else {
+    Function("r", "regeneratorRuntime = r")(runtime);
+  }
+}