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/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" />