v1.4.3

feat: Add Amazon Nova Reel video generation support

- Add NovaReelModels.swift with task types (Text to Video, Multi-Shot Auto/Manual)
- Add VideoGenerationService.swift with async invoke API and S3 download
- Add NovaReelConfigDropdown.swift for video generation settings UI
- Add GeneratedVideoView in MessageView for inline video playback
- Add video polling with real-time status updates in chat
- Add sidebar loading indicator during video generation
- Store videoUrl/videoS3Uri in Message and MessageData for persistence
- Support video playback on chat reload
- Update README with video generation feature