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/leven/index.d.ts

@@ -0,0 +1,21 @@
+declare const leven: {
+	/**
+	Measure the difference between two strings.
+
+	@example
+	```
+	import leven = require('leven');
+
+	leven('cat', 'cow');
+	//=> 2
+	```
+	*/
+	(left: string, right: string): number;
+
+	// TODO: Remove this for the next major release, refactor the whole definition to:
+	// declare function leven(left: string, right: string): number;
+	// export = leven;
+	default: typeof leven;
+};
+
+export = leven;