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/estree-walker/src/index.js

@@ -0,0 +1,35 @@
+// @ts-check
+import { SyncWalker } from './sync.js';
+import { AsyncWalker } from './async.js';
+
+/** @typedef { import('estree').BaseNode} BaseNode */
+/** @typedef { import('./sync.js').SyncHandler} SyncHandler */
+/** @typedef { import('./async.js').AsyncHandler} AsyncHandler */
+
+/**
+ *
+ * @param {BaseNode} ast
+ * @param {{
+ *   enter?: SyncHandler
+ *   leave?: SyncHandler
+ * }} walker
+ * @returns {BaseNode}
+ */
+export function walk(ast, { enter, leave }) {
+	const instance = new SyncWalker(enter, leave);
+	return instance.visit(ast, null);
+}
+
+/**
+ *
+ * @param {BaseNode} ast
+ * @param {{
+ *   enter?: AsyncHandler
+ *   leave?: AsyncHandler
+ * }} walker
+ * @returns {Promise<BaseNode>}
+ */
+export async function asyncWalk(ast, { enter, leave }) {
+	const instance = new AsyncWalker(enter, leave);
+	return await instance.visit(ast, null);
+}