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/workbox-build/build/lib/stringify-without-comments.js

@@ -0,0 +1,28 @@
+"use strict";
+/*
+  Copyright 2021 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+var __importDefault = (this && this.__importDefault) || function (mod) {
+    return (mod && mod.__esModule) ? mod : { "default": mod };
+};
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.stringifyWithoutComments = void 0;
+const stringify_object_1 = __importDefault(require("stringify-object"));
+const strip_comments_1 = __importDefault(require("strip-comments"));
+function stringifyWithoutComments(obj) {
+    return (0, stringify_object_1.default)(obj, {
+        // See https://github.com/yeoman/stringify-object#transformobject-property-originalresult
+        transform: (_obj, _prop, str) => {
+            if (typeof _prop !== 'symbol' && typeof _obj[_prop] === 'function') {
+                // Can't typify correctly stripComments
+                return (0, strip_comments_1.default)(str); // eslint-disable-line
+            }
+            return str;
+        },
+    });
+}
+exports.stringifyWithoutComments = stringifyWithoutComments;