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/resolve/example/async.js

@@ -0,0 +1,5 @@
+var resolve = require('../');
+resolve('tap', { basedir: __dirname }, function (err, res) {
+    if (err) console.error(err);
+    else console.log(res);
+});

frontend/node_modules/resolve/example/sync.js

@@ -0,0 +1,3 @@
+var resolve = require('../');
+var res = resolve.sync('tap', { basedir: __dirname });
+console.log(res);