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/test/subdirs.js

@@ -0,0 +1,13 @@
+var test = require('tape');
+var resolve = require('../');
+var path = require('path');
+
+test('subdirs', function (t) {
+    t.plan(2);
+
+    var dir = path.join(__dirname, '/subdirs');
+    resolve('a/b/c/x.json', { basedir: dir }, function (err, res) {
+        t.ifError(err);
+        t.equal(res, path.join(dir, 'node_modules/a/b/c/x.json'));
+    });
+});