Kanban Board with Drag and Drop

Create a kanban board with:
- 4 columns: "Backlog", "In Progress", "In Review", "Done"
- Each column has a header with title, item count badge, and add card button
- Cards with: title, assignee avatar, priority badge (High/Med/Low), due date
- Cards are draggable between columns
- Drag state: card becomes semi-transparent, drop zone highlighted
- Add new card: click + button opens an inline form at bottom of column
- Column scroll independently when content overflows
- "Add Column" button at the end
- Use Tailwind CSS, @hello-pangea/dnd for drag and drop
- Dark mode support