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/fast-json-stable-stringify/test/nested.js

@@ -0,0 +1,44 @@
+'use strict';
+
+var test = require('tape');
+var stringify = require('../');
+
+test('nested', function (t) {
+    t.plan(1);
+    var obj = { c: 8, b: [{z:6,y:5,x:4},7], a: 3 };
+    t.equal(stringify(obj), '{"a":3,"b":[{"x":4,"y":5,"z":6},7],"c":8}');
+});
+
+test('cyclic (default)', function (t) {
+    t.plan(1);
+    var one = { a: 1 };
+    var two = { a: 2, one: one };
+    one.two = two;
+    try {
+        stringify(one);
+    } catch (ex) {
+        t.equal(ex.toString(), 'TypeError: Converting circular structure to JSON');
+    }
+});
+
+test('cyclic (specifically allowed)', function (t) {
+    t.plan(1);
+    var one = { a: 1 };
+    var two = { a: 2, one: one };
+    one.two = two;
+    t.equal(stringify(one, {cycles:true}), '{"a":1,"two":{"a":2,"one":"__cycle__"}}');
+});
+
+test('repeated non-cyclic value', function(t) {
+    t.plan(1);
+    var one = { x: 1 };
+    var two = { a: one, b: one };
+    t.equal(stringify(two), '{"a":{"x":1},"b":{"x":1}}');
+});
+
+test('acyclic but with reused obj-property pointers', function (t) {
+    t.plan(1);
+    var x = { a: 1 };
+    var y = { b: x, c: x };
+    t.equal(stringify(y), '{"b":{"a":1},"c":{"a":1}}');
+});