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/rollup/dist/loadConfigFile.d.ts

@@ -0,0 +1,20 @@
+import type { LogHandler, MergedRollupOptions, RollupLog } from './rollup';
+
+export interface BatchWarnings {
+	add: (warning: RollupLog) => void;
+	readonly count: number;
+	flush: () => void;
+	log: LogHandler;
+	readonly warningOccurred: boolean;
+}
+
+export type LoadConfigFile = typeof loadConfigFile;
+
+export function loadConfigFile(
+	fileName: string,
+	commandOptions: any,
+	watchMode?: boolean
+): Promise<{
+	options: MergedRollupOptions[];
+	warnings: BatchWarnings;
+}>;