fix: address code review findings — data loss, race condition, broken features

- TopBar: call closeBoard() before navigating back to prevent data loss
- board-store: guard debouncedSave against race condition when board is
  closed during an in-flight save
- board-store: add missing updatedAt to setColumnWidth
- useKeyboardShortcuts: remove duplicate Ctrl+K handler that prevented
  command palette from toggling closed
- AttachmentSection: wire up Tauri file dialog for adding attachments
  with link/copy mode support

src/components/card-detail/AttachmentSection.tsx

@@ -1,6 +1,8 @@
 import { FileIcon, X, Plus } from "lucide-react";
+import { open } from "@tauri-apps/plugin-dialog";
 import { Button } from "@/components/ui/button";
 import { useBoardStore } from "@/stores/board-store";
+import { copyAttachment } from "@/lib/storage";
 import type { Attachment } from "@/types/board";
 
 interface AttachmentSectionProps {
@@ -12,11 +14,29 @@ export function AttachmentSection({
   cardId,
   attachments,
 }: AttachmentSectionProps) {
+  const addAttachment = useBoardStore((s) => s.addAttachment);
   const removeAttachment = useBoardStore((s) => s.removeAttachment);
 
-  function handleAdd() {
-    // Placeholder: Tauri file dialog will be wired in a later task
-    console.log("Add attachment (file dialog not yet wired)");
+  async function handleAdd() {
+    const selected = await open({
+      multiple: false,
+      title: "Select attachment",
+    });
+    if (!selected) return;
+
+    const fileName = selected.split(/[\\/]/).pop() ?? "attachment";
+
+    const board = useBoardStore.getState().board;
+    if (!board) return;
+
+    const mode = board.settings.attachmentMode;
+
+    if (mode === "copy") {
+      const destPath = await copyAttachment(board.id, selected, fileName);
+      addAttachment(cardId, { name: fileName, path: destPath, mode: "copy" });
+    } else {
+      addAttachment(cardId, { name: fileName, path: selected, mode: "link" });
+    }
   }
 
   return (

src/components/layout/TopBar.tsx

@@ -75,7 +75,10 @@ export function TopBar() {
               <Button
                 variant="ghost"
                 size="sm"
-                onClick={() => setView({ type: "board-list" })}
+                onClick={() => {
+                  useBoardStore.getState().closeBoard();
+                  setView({ type: "board-list" });
+                }}
                 className="text-pylon-text-secondary hover:text-pylon-text"
               >
                 <ArrowLeft className="size-4" />