Tech: remove old attachment components

Remove deprecated components:
- UniqueComponent and MultipleComponent
- DragDropUploaderComponent
- ButtonUploaderComponent (replaced by FileInputComponent)
- drag_drop_uploader_controller.ts

All usages have been migrated to the new unified architecture.

Part of attachment components refactoring (#7/7)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mfo