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-recipes/warmStrategyCache.js

@@ -0,0 +1,18 @@
+import './_version.js';
+/**
+ * @memberof workbox-recipes
+ 
+ * @param {Object} options
+ * @param {string[]} options.urls Paths to warm the strategy's cache with
+ * @param {Strategy} options.strategy Strategy to use
+ */
+function warmStrategyCache(options) {
+    self.addEventListener('install', (event) => {
+        const done = options.urls.map((path) => options.strategy.handleAll({
+            event,
+            request: new Request(path),
+        })[1]);
+        event.waitUntil(Promise.all(done));
+    });
+}
+export { warmStrategyCache };