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/es-to-primitive/eslint.config.mjs

@@ -0,0 +1,29 @@
+import ljharb from '@ljharb/eslint-config/flat';
+
+export default [
+	...ljharb,
+	{
+		rules: {
+			'func-style': [
+				'error',
+				'declaration',
+			],
+			'id-length': [
+				'error', {
+					max: 24,
+					min: 1,
+					properties: 'never',
+				},
+			],
+			'new-cap': [
+				'error', {
+					capIsNewExceptions: [
+						'GetMethod',
+						'OrdinaryToPrimitive',
+					],
+				},
+			],
+			'no-extra-parens': 'off',
+		},
+	},
+];