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/source-map/lib/read-wasm-browser.js

@@ -0,0 +1,22 @@
+"use strict";
+
+let mappingsWasm = null;
+
+module.exports = function readWasm() {
+  if (typeof mappingsWasm === "string") {
+    return fetch(mappingsWasm)
+      .then(response => response.arrayBuffer());
+  }
+  if (mappingsWasm instanceof ArrayBuffer) {
+    return Promise.resolve(mappingsWasm);
+  }
+
+  throw new Error("You must provide the string URL or ArrayBuffer contents " +
+                  "of lib/mappings.wasm by calling " +
+                  "SourceMapConsumer.initialize({ 'lib/mappings.wasm': ... }) " +
+                  "before using SourceMapConsumer");
+};
+
+module.exports.initialize = input => {
+  mappingsWasm = input;
+};