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/BoardView.tsx

@@ -23,6 +23,7 @@ import {
   CardOverlay,
   ColumnOverlay,
 } from "@/components/board/DragOverlayContent";
+import { CardDetailModal } from "@/components/card-detail/CardDetailModal";
 import type { Board } from "@/types/board";
 
 function findColumnByCardId(board: Board, cardId: string) {
@@ -35,6 +36,7 @@ export function BoardView() {
   const moveCard = useBoardStore((s) => s.moveCard);
   const moveColumn = useBoardStore((s) => s.moveColumn);
 
+  const [selectedCardId, setSelectedCardId] = useState<string | null>(null);
   const [addingColumn, setAddingColumn] = useState(false);
   const [newColumnTitle, setNewColumnTitle] = useState("");
   const inputRef = useRef<HTMLInputElement>(null);
@@ -233,6 +235,7 @@ export function BoardView() {
   const columnIds = board.columns.map((c) => c.id);
 
   return (
+    <>
     <DndContext
       sensors={sensors}
       collisionDetection={closestCorners}
@@ -246,7 +249,11 @@ export function BoardView() {
       >
         <div className="flex h-full gap-6 overflow-x-auto p-6">
           {board.columns.map((column) => (
-            <KanbanColumn key={column.id} column={column} />
+            <KanbanColumn
+              key={column.id}
+              column={column}
+              onCardClick={setSelectedCardId}
+            />
           ))}
 
           {/* Add column button / inline input */}
@@ -307,5 +314,11 @@ export function BoardView() {
         ) : null}
       </DragOverlay>
     </DndContext>
+
+    <CardDetailModal
+      cardId={selectedCardId}
+      onClose={() => setSelectedCardId(null)}
+    />
+    </>
   );
 }