feat: add two-panel card detail modal with markdown, checklist, labels, dates, attachments

- CardDetailModal: two-panel layout (60/40) with inline title editing
- MarkdownEditor: edit/preview toggle with react-markdown + remark-gfm
- ChecklistSection: add/toggle/edit/delete items with progress counter
- LabelPicker: toggle labels + create new labels with color swatches
- DueDatePicker: date input with relative time and overdue styling
- AttachmentSection: list with remove, placeholder add button
- Wired into BoardView via selectedCardId state

src/components/board/KanbanColumn.tsx

@@ -23,9 +23,10 @@ const WIDTH_MAP = {
 
 interface KanbanColumnProps {
   column: Column;
+  onCardClick?: (cardId: string) => void;
 }
 
-export function KanbanColumn({ column }: KanbanColumnProps) {
+export function KanbanColumn({ column, onCardClick }: KanbanColumnProps) {
   const [showAddCard, setShowAddCard] = useState(false);
   const board = useBoardStore((s) => s.board);
 
@@ -85,6 +86,7 @@ export function KanbanColumn({ column }: KanbanColumnProps) {
                   card={card}
                   boardLabels={board?.labels ?? []}
                   columnId={column.id}
+                  onCardClick={onCardClick}
                 />
               );
             })}