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/deepmerge/rollup.config.js

@@ -0,0 +1,22 @@
+import resolve from 'rollup-plugin-node-resolve'
+import commonjs from 'rollup-plugin-commonjs'
+import pkg from './package.json'
+
+export default {
+	input: `index.js`,
+	plugins: [
+		commonjs(),
+		resolve(),
+	],
+	output: [
+		{
+			file: pkg.main,
+			format: `cjs`
+		},
+		{
+			name: 'deepmerge',
+			file: 'dist/umd.js',
+			format: `umd`
+		},
+	],
+}