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/vite/dist/client/env.mjs

@@ -0,0 +1,24 @@
+const context = (() => {
+  if (typeof globalThis !== "undefined") {
+    return globalThis;
+  } else if (typeof self !== "undefined") {
+    return self;
+  } else if (typeof window !== "undefined") {
+    return window;
+  } else {
+    return Function("return this")();
+  }
+})();
+const defines = __DEFINES__;
+Object.keys(defines).forEach((key) => {
+  const segments = key.split(".");
+  let target = context;
+  for (let i = 0; i < segments.length; i++) {
+    const segment = segments[i];
+    if (i === segments.length - 1) {
+      target[segment] = defines[key];
+    } else {
+      target = target[segment] || (target[segment] = {});
+    }
+  }
+});