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/foreground-child/dist/commonjs/proxy-signals.js

@@ -0,0 +1,38 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.proxySignals = void 0;
+const all_signals_js_1 = require("./all-signals.js");
+/**
+ * Starts forwarding signals to `child` through `parent`.
+ */
+const proxySignals = (child) => {
+    const listeners = new Map();
+    for (const sig of all_signals_js_1.allSignals) {
+        const listener = () => {
+            // some signals can only be received, not sent
+            try {
+                child.kill(sig);
+                /* c8 ignore start */
+            }
+            catch (_) { }
+            /* c8 ignore stop */
+        };
+        try {
+            // if it's a signal this system doesn't recognize, skip it
+            process.on(sig, listener);
+            listeners.set(sig, listener);
+            /* c8 ignore start */
+        }
+        catch (_) { }
+        /* c8 ignore stop */
+    }
+    const unproxy = () => {
+        for (const [sig, listener] of listeners) {
+            process.removeListener(sig, listener);
+        }
+    };
+    child.on('exit', unproxy);
+    return unproxy;
+};
+exports.proxySignals = proxySignals;
+//# sourceMappingURL=proxy-signals.js.map