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/async/internal/asyncEachOfLimit.js

@@ -0,0 +1,75 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+exports.default = asyncEachOfLimit;
+
+var _breakLoop = require('./breakLoop.js');
+
+var _breakLoop2 = _interopRequireDefault(_breakLoop);
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+// for async generators
+function asyncEachOfLimit(generator, limit, iteratee, callback) {
+    let done = false;
+    let canceled = false;
+    let awaiting = false;
+    let running = 0;
+    let idx = 0;
+
+    function replenish() {
+        //console.log('replenish')
+        if (running >= limit || awaiting || done) return;
+        //console.log('replenish awaiting')
+        awaiting = true;
+        generator.next().then(({ value, done: iterDone }) => {
+            //console.log('got value', value)
+            if (canceled || done) return;
+            awaiting = false;
+            if (iterDone) {
+                done = true;
+                if (running <= 0) {
+                    //console.log('done nextCb')
+                    callback(null);
+                }
+                return;
+            }
+            running++;
+            iteratee(value, idx, iterateeCallback);
+            idx++;
+            replenish();
+        }).catch(handleError);
+    }
+
+    function iterateeCallback(err, result) {
+        //console.log('iterateeCallback')
+        running -= 1;
+        if (canceled) return;
+        if (err) return handleError(err);
+
+        if (err === false) {
+            done = true;
+            canceled = true;
+            return;
+        }
+
+        if (result === _breakLoop2.default || done && running <= 0) {
+            done = true;
+            //console.log('done iterCb')
+            return callback(null);
+        }
+        replenish();
+    }
+
+    function handleError(err) {
+        if (canceled) return;
+        awaiting = false;
+        done = true;
+        callback(err);
+    }
+
+    replenish();
+}
+module.exports = exports.default;