* feat: support Batch API for retain (openai/groq)
* api
* stop batch api if sync
* fix(ui): improve toast notifications with brand colors and proper styling
- Replace all window.alert() calls with toast notifications
- Add interceptor-based error handling in API client
- Use different toast styles based on HTTP status codes (4xx = warning, 5xx = error)
- Apply Hindsight brand colors to toasts (primary blue for info, destructive red for errors, etc.)
- Remove obsolete error handling files (hindsight-client-with-toast.ts, api-error-handler.ts)
- Fix toast background conflicts by removing base bg-background class
* fix: restore retain_batch_tokens config that was accidentally removed during rebase
