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/board/BoardView.tsx

@@ -230,24 +230,6 @@ export function BoardView() {
     [board, moveCard, moveColumn]
   );
 
-  if (!board) {
-    return (
-      <div className="flex h-full items-center justify-center text-pylon-text-secondary">
-        <span className="font-mono text-sm">Loading board...</span>
-      </div>
-    );
-  }
-
-  // Get the active item for the drag overlay
-  const activeCard =
-    activeType === "card" && activeId ? board.cards[activeId] : null;
-  const activeColumn =
-    activeType === "column" && activeId
-      ? board.columns.find((c) => c.id === activeId)
-      : null;
-
-  const columnIds = board.columns.map((c) => c.id);
-
   const [announcement, setAnnouncement] = useState("");
 
   const handleDragEndWithAnnouncement = useCallback(
@@ -275,6 +257,24 @@ export function BoardView() {
     [handleDragEnd, board]
   );
 
+  if (!board) {
+    return (
+      <div className="flex h-full items-center justify-center text-pylon-text-secondary">
+        <span className="font-mono text-sm">Loading board...</span>
+      </div>
+    );
+  }
+
+  // Get the active item for the drag overlay
+  const activeCard =
+    activeType === "card" && activeId ? board.cards[activeId] : null;
+  const activeColumn =
+    activeType === "column" && activeId
+      ? board.columns.find((c) => c.id === activeId)
+      : null;
+
+  const columnIds = board.columns.map((c) => c.id);
+
   return (
     <>
     {/* Visually hidden live region for drag-and-drop announcements */}