fix: move hooks before early return in BoardView, remove unused attachmentMode prop

Fixed React hooks rules violation where useState and useCallback were
called after a conditional return in BoardView. Removed unused
attachmentMode prop from AttachmentSection (can be re-added when file
dialog is wired up).

src/components/card-detail/AttachmentSection.tsx

@@ -6,7 +6,6 @@ import type { Attachment } from "@/types/board";
 interface AttachmentSectionProps {
   cardId: string;
   attachments: Attachment[];
-  attachmentMode: "link" | "copy";
 }
 
 export function AttachmentSection({