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/array-buffer-byte-length/test/index.js

@@ -0,0 +1,29 @@
+'use strict';
+
+var test = require('tape');
+var forEach = require('for-each');
+var v = require('es-value-fixtures');
+var inspect = require('object-inspect');
+
+var byteLength = require('../');
+
+test('byteLength', function (t) {
+	forEach(v.objects.concat(v.primitives), function (nonAB) {
+		t.equal(byteLength(nonAB), NaN, inspect(nonAB) + ' is not an ArrayBuffer, and yields NaN');
+	});
+
+	t.test('ArrayBuffers', { skip: typeof ArrayBuffer !== 'function' }, function (st) {
+		var ab32 = new ArrayBuffer(32);
+		st.equal(byteLength(ab32), 32, 'works on an ArrayBuffer of length 32: ' + inspect(ab32));
+
+		var ab0 = new ArrayBuffer(0);
+		st.equal(byteLength(ab0), 0, 'works on an ArrayBuffer of length 0: ' + inspect(ab0));
+
+		var dv = new DataView(ab32);
+		st.equal(byteLength(dv), NaN, 'a DataView returns NaN');
+
+		st.end();
+	});
+
+	t.end();
+});