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/CardThumbnail.tsx

@@ -9,9 +9,10 @@ interface CardThumbnailProps {
   card: Card;
   boardLabels: Label[];
   columnId: string;
+  onCardClick?: (cardId: string) => void;
 }
 
-export function CardThumbnail({ card, boardLabels, columnId }: CardThumbnailProps) {
+export function CardThumbnail({ card, boardLabels, columnId, onCardClick }: CardThumbnailProps) {
   const {
     attributes,
     listeners,
@@ -35,8 +36,7 @@ export function CardThumbnail({ card, boardLabels, columnId }: CardThumbnailProp
   const overdue = dueDate != null && isPast(dueDate) && !isToday(dueDate);
 
   function handleClick() {
-    // Card detail modal will be wired in Task 11
-    console.log("Card clicked:", card.id);
+    onCardClick?.(card.id);
   }
 
   return (