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/react-dom/profiling.js

@@ -0,0 +1,38 @@
+'use strict';
+
+function checkDCE() {
+  /* global __REACT_DEVTOOLS_GLOBAL_HOOK__ */
+  if (
+    typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ === 'undefined' ||
+    typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.checkDCE !== 'function'
+  ) {
+    return;
+  }
+  if (process.env.NODE_ENV !== 'production') {
+    // This branch is unreachable because this function is only called
+    // in production, but the condition is true only in development.
+    // Therefore if the branch is still here, dead code elimination wasn't
+    // properly applied.
+    // Don't change the message. React DevTools relies on it. Also make sure
+    // this message doesn't occur elsewhere in this function, or it will cause
+    // a false positive.
+    throw new Error('^_^');
+  }
+  try {
+    // Verify that the code above has been dead code eliminated (DCE'd).
+    __REACT_DEVTOOLS_GLOBAL_HOOK__.checkDCE(checkDCE);
+  } catch (err) {
+    // DevTools shouldn't crash React, no matter what.
+    // We should still report in case we break this code.
+    console.error(err);
+  }
+}
+
+if (process.env.NODE_ENV === 'production') {
+  // DCE check should happen before ReactDOM bundle executes so that
+  // DevTools can report bad minification during injection.
+  checkDCE();
+  module.exports = require('./cjs/react-dom.profiling.min.js');
+} else {
+  module.exports = require('./cjs/react-dom.development.js');
+}